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SQL As Understood By SQLite sqlite.orgThe SQLite library understands most of the standard SQL language. But it does omit some features while at the same time adding a few features of its own. This document attempts to describe precisely what parts of the SQL language SQLite does and does not support. A list of keywords is also provided. In all of the syntax diagrams that follow, literal text is shown in bold blue. Non-terminal symbols are shown in italic red. Operators that are part of the syntactic markup itself are shown in black roman. This document is just an overview of the SQL syntax implemented by SQLite. Many low-level productions are omitted. For detailed information on the language that SQLite understands, refer to the source code and the grammar file "parse.y". SQLite implements the follow syntax:
Details on the implementation of each command are provided in the sequel. ALTER TABLE
SQLite's version of the ALTER TABLE command allows the user to rename or add a new column to an existing table. It is not possible to remove a column from a table. The RENAME TO syntax is used to rename the table identified by [database-name.]table-name to new-table-name. This command cannot be used to move a table between attached databases, only to rename a table within the same database. If the table being renamed has triggers or indices, then these remain attached to the table after it has been renamed. However, if there are any view definitions, or statements executed by triggers that refer to the table being renamed, these are not automatically modified to use the new table name. If this is required, the triggers or view definitions must be dropped and recreated to use the new table name by hand. The ADD [COLUMN] syntax is used to add a new column to an existing table. The new column is always appended to the end of the list of existing columns. Column-def may take any of the forms permissable in a CREATE TABLE statement, with the following restrictions:
The execution time of the ALTER TABLE command is independent of the amount of data in the table. The ALTER TABLE command runs as quickly on a table with 10 million rows as it does on a table with 1 row. After ADD COLUMN has been run on a database, that database will not be readable by SQLite version 3.1.3 and earlier until the database is VACUUMed. ANALYZE
The ANALYZE command gathers statistics about indices and stores them in a special tables in the database where the query optimizer can use them to help make better index choices. If no arguments are given, all indices in all attached databases are analyzed. If a database name is given as the argument, all indices in that one database are analyzed. If the argument is a table name, then only indices associated with that one table are analyzed. The initial implementation stores all statistics in a single table named sqlite_stat1. Future enhancements may create additional tables with the same name pattern except with the "1" changed to a different digit. The sqlite_stat1 table cannot be DROPped, but it all the content can be DELETEd which has the same effect. ATTACH DATABASE
The ATTACH DATABASE statement adds a preexisting database file to the current database connection. If the filename contains punctuation characters it must be quoted. The names 'main' and 'temp' refer to the main database and the database used for temporary tables. These cannot be detached. Attached databases are removed using the DETACH DATABASE statement. You can read from and write to an attached database and you can modify the schema of the attached database. This is a new feature of SQLite version 3.0. In SQLite 2.8, schema changes to attached databases were not allowed. You cannot create a new table with the same name as a table in an attached database, but you can attach a database which contains tables whose names are duplicates of tables in the main database. It is also permissible to attach the same database file multiple times. Tables in an attached database can be referred to using the syntax database-name.table-name. If an attached table doesn't have a duplicate table name in the main database, it doesn't require a database name prefix. When a database is attached, all of its tables which don't have duplicate names become the 'default' table of that name. Any tables of that name attached afterwards require the table prefix. If the 'default' table of a given name is detached, then the last table of that name attached becomes the new default. Transactions involving multiple attached databases are atomic, assuming that the main database is not ":memory:". If the main database is ":memory:" then transactions continue to be atomic within each individual database file. But if the host computer crashes in the middle of a COMMIT where two or more database files are updated, some of those files might get the changes where others might not. Atomic commit of attached databases is a new feature of SQLite version 3.0. In SQLite version 2.8, all commits to attached databases behaved as if the main database were ":memory:". There is a compile-time limit of 10 attached database files. BEGIN TRANSACTION
Beginning in version 2.0, SQLite supports transactions with rollback and atomic commit. The optional transaction name is ignored. SQLite currently does not allow nested transactions. No changes can be made to the database except within a transaction. Any command that changes the database (basically, any SQL command other than SELECT) will automatically start a transaction if one is not already in effect. Automatically started transactions are committed at the conclusion of the command. Transactions can be started manually using the BEGIN command. Such transactions usually persist until the next COMMIT or ROLLBACK command. But a transaction will also ROLLBACK if the database is closed or if an error occurs and the ROLLBACK conflict resolution algorithm is specified. See the documentation on the ON CONFLICT clause for additional information about the ROLLBACK conflict resolution algorithm. In SQLite version 3.0.8 and later, transactions can be deferred, immediate, or exclusive. Deferred means that no locks are acquired on the database until the database is first accessed. Thus with a deferred transaction, the BEGIN statement itself does nothing. Locks are not acquired until the first read or write operation. The first read operation against a database creates a SHARED lock and the first write operation creates a RESERVED lock. Because the acquisition of locks is deferred until they are needed, it is possible that another thread or process could create a separate transaction and write to the database after the BEGIN on the current thread has executed. If the transaction is immediate, then RESERVED locks are acquired on all databases as soon as the BEGIN command is executed, without waiting for the database to be used. After a BEGIN IMMEDIATE, you are guaranteed that no other thread or process will be able to write to the database or do a BEGIN IMMEDIATE or BEGIN EXCLUSIVE. Other processes can continue to read from the database, however. An exclusive transaction causes EXCLUSIVE locks to be acquired on all databases. After a BEGIN EXCLUSIVE, you are guaranteed that no other thread or process will be able to read or write the database until the transaction is complete. A description of the meaning of SHARED, RESERVED, and EXCLUSIVE locks is available separately. The default behavior for SQLite version 3.0.8 is a deferred transaction. For SQLite version 3.0.0 through 3.0.7, deferred is the only kind of transaction available. For SQLite version 2.8 and earlier, all transactions are exclusive. The COMMIT command does not actually perform a commit until all pending SQL commands finish. Thus if two or more SELECT statements are in the middle of processing and a COMMIT is executed, the commit will not actually occur until all SELECT statements finish. An attempt to execute COMMIT might result in an SQLITE_BUSY return code. This indicates that another thread or process had a read lock on the database that prevented the database from being updated. When COMMIT fails in this way, the transaction remains active and the COMMIT can be retried later after the reader has had a chance to clear. comment
Comments aren't SQL commands, but can occur in SQL queries. They are treated as whitespace by the parser. They can begin anywhere whitespace can be found, including inside expressions that span multiple lines. SQL comments only extend to the end of the current line. C comments can span any number of lines. If there is no terminating delimiter, they extend to the end of the input. This is not treated as an error. A new SQL statement can begin on a line after a multiline comment ends. C comments can be embedded anywhere whitespace can occur, including inside expressions, and in the middle of other SQL statements. C comments do not nest. SQL comments inside a C comment will be ignored. COPY
The COPY command is available in SQLite version 2.8 and earlier. The COPY command has been removed from SQLite version 3.0 due to complications in trying to support it in a mixed UTF-8/16 environment. In version 3.0, the command-line shell contains a new command .import that can be used as a substitute for COPY. The COPY command is an extension used to load large amounts of data into a table. It is modeled after a similar command found in PostgreSQL. In fact, the SQLite COPY command is specifically designed to be able to read the output of the PostgreSQL dump utility pg_dump so that data can be easily transferred from PostgreSQL into SQLite. The table-name is the name of an existing table which is to be filled with data. The filename is a string or identifier that names a file from which data will be read. The filename can be the STDIN to read data from standard input. Each line of the input file is converted into a single record in the table. Columns are separated by tabs. If a tab occurs as data within a column, then that tab is preceded by a baskslash "\" character. A baskslash in the data appears as two backslashes in a row. The optional USING DELIMITERS clause can specify a delimiter other than tab. If a column consists of the character "\N", that column is filled with the value NULL. The optional conflict-clause allows the specification of an alternative constraint conflict resolution algorithm to use for this one command. See the section titled ON CONFLICT for additional information. When the input data source is STDIN, the input can be terminated by a line that contains only a baskslash and a dot: "\.". CREATE INDEX
The CREATE INDEX command consists of the keywords "CREATE INDEX" followed by the name of the new index, the keyword "ON", the name of a previously created table that is to be indexed, and a parenthesized list of names of columns in the table that are used for the index key. Each column name can be followed by one of the "ASC" or "DESC" keywords to indicate sort order, but the sort order is ignored in the current implementation. Sorting is always done in ascending order. The COLLATE clause following each column name defines a collating sequence used for text entires in that column. The default collating sequence is the collating sequence defined for that column in the CREATE TABLE statement. Or if no collating sequence is otherwise defined, the built-in BINARY collating sequence is used. There are no arbitrary limits on the number of indices that can be attached to a single table, nor on the number of columns in an index. If the UNIQUE keyword appears between CREATE and INDEX then duplicate index entries are not allowed. Any attempt to insert a duplicate entry will result in an error. The exact text of each CREATE INDEX statement is stored in the sqlite_master or sqlite_temp_master table, depending on whether the table being indexed is temporary. Every time the database is opened, all CREATE INDEX statements are read from the sqlite_master table and used to regenerate SQLite's internal representation of the index layout. If the optional IF NOT EXISTS clause is present and another index with the same name aleady exists, then this command becomes a no-op. Indexes are removed with the DROP INDEX command. CREATE TABLE
A CREATE TABLE statement is basically the keywords "CREATE TABLE" followed by the name of a new table and a parenthesized list of column definitions and constraints. The table name can be either an identifier or a string. Tables names that begin with "sqlite_" are reserved for use by the engine. Each column definition is the name of the column followed by the datatype for that column, then one or more optional column constraints. The datatype for the column does not restrict what data may be put in that column. See Datatypes In SQLite Version 3 for additional information. The UNIQUE constraint causes an index to be created on the specified columns. This index must contain unique keys. The COLLATE clause specifies what text collating function to use when comparing text entries for the column. The built-in BINARY collating function is used by default. The DEFAULT constraint specifies a default value to use when doing an INSERT. The value may be NULL, a string constant or a number. Starting with version 3.1.0, the default value may also be one of the special case-independant keywords CURRENT_TIME, CURRENT_DATE or CURRENT_TIMESTAMP. If the value is NULL, a string constant or number, it is literally inserted into the column whenever an INSERT statement that does not specify a value for the column is executed. If the value is CURRENT_TIME, CURRENT_DATE or CURRENT_TIMESTAMP, then the current UTC date and/or time is inserted into the columns. For CURRENT_TIME, the format is HH:MM:SS. For CURRENT_DATE, YYYY-MM-DD. The format for CURRENT_TIMESTAMP is "YYYY-MM-DD HH:MM:SS". Specifying a PRIMARY KEY normally just creates a UNIQUE index on the corresponding columns. However, if primary key is on a single column that has datatype INTEGER, then that column is used internally as the actual key of the B-Tree for the table. This means that the column may only hold unique integer values. (Except for this one case, SQLite ignores the datatype specification of columns and allows any kind of data to be put in a column regardless of its declared datatype.) If a table does not have an INTEGER PRIMARY KEY column, then the B-Tree key will be a automatically generated integer. The B-Tree key for a row can always be accessed using one of the special names "ROWID", "OID", or "_ROWID_". This is true regardless of whether or not there is an INTEGER PRIMARY KEY. An INTEGER PRIMARY KEY column man also include the keyword AUTOINCREMENT. The AUTOINCREMENT keyword modified the way that B-Tree keys are automatically generated. Additional detail on automatic B-Tree key generation is available separately. If the "TEMP" or "TEMPORARY" keyword occurs in between "CREATE" and "TABLE" then the table that is created is only visible within that same database connection and is automatically deleted when the database connection is closed. Any indices created on a temporary table are also temporary. Temporary tables and indices are stored in a separate file distinct from the main database file. If a <database-name> is specified, then the table is created in the named database. It is an error to specify both a <database-name> and the TEMP keyword, unless the <database-name> is "temp". If no database name is specified, and the TEMP keyword is not present, the table is created in the main database. The optional conflict-clause following each constraint allows the specification of an alternative default constraint conflict resolution algorithm for that constraint. The default is abort ABORT. Different constraints within the same table may have different default conflict resolution algorithms. If an COPY, INSERT, or UPDATE command specifies a different conflict resolution algorithm, then that algorithm is used in place of the default algorithm specified in the CREATE TABLE statement. See the section titled ON CONFLICT for additional information. CHECK constraints are supported as of version 3.3.0. Prior to version 3.3.0, CHECK constraints were parsed but not enforced. There are no arbitrary limits on the number of columns or on the number of constraints in a table. The total amount of data in a single row is limited to about 1 megabytes in version 2.8. In version 3.0 there is no arbitrary limit on the amount of data in a row. The CREATE TABLE AS form defines the table to be the result set of a query. The names of the table columns are the names of the columns in the result. The exact text of each CREATE TABLE statement is stored in the sqlite_master table. Every time the database is opened, all CREATE TABLE statements are read from the sqlite_master table and used to regenerate SQLite's internal representation of the table layout. If the original command was a CREATE TABLE AS then then an equivalent CREATE TABLE statement is synthesized and store in sqlite_master in place of the original command. The text of CREATE TEMPORARY TABLE statements are stored in the sqlite_temp_master table. If the optional IF NOT EXISTS clause is present and another table with the same name aleady exists, then this command becomes a no-op. Tables are removed using the DROP TABLE statement. CREATE TRIGGER
The CREATE TRIGGER statement is used to add triggers to the database schema. Triggers are database operations (the trigger-action) that are automatically performed when a specified database event (the database-event) occurs. A trigger may be specified to fire whenever a DELETE, INSERT or UPDATE of a particular database table occurs, or whenever an UPDATE of one or more specified columns of a table are updated. At this time SQLite supports only FOR EACH ROW triggers, not FOR EACH STATEMENT triggers. Hence explicitly specifying FOR EACH ROW is optional. FOR EACH ROW implies that the SQL statements specified as trigger-steps may be executed (depending on the WHEN clause) for each database row being inserted, updated or deleted by the statement causing the trigger to fire. Both the WHEN clause and the trigger-steps may access elements of the row being inserted, deleted or updated using references of the form "NEW.column-name" and "OLD.column-name", where column-name is the name of a column from the table that the trigger is associated with. OLD and NEW references may only be used in triggers on trigger-events for which they are relevant, as follows:
If a WHEN clause is supplied, the SQL statements specified as trigger-steps are only executed for rows for which the WHEN clause is true. If no WHEN clause is supplied, the SQL statements are executed for all rows. The specified trigger-time determines when the trigger-steps will be executed relative to the insertion, modification or removal of the associated row. An ON CONFLICT clause may be specified as part of an UPDATE or INSERT trigger-step. However if an ON CONFLICT clause is specified as part of the statement causing the trigger to fire, then this conflict handling policy is used instead. Triggers are automatically dropped when the table that they are associated with is dropped. Triggers may be created on views, as well as ordinary tables, by specifying INSTEAD OF in the CREATE TRIGGER statement. If one or more ON INSERT, ON DELETE or ON UPDATE triggers are defined on a view, then it is not an error to execute an INSERT, DELETE or UPDATE statement on the view, respectively. Thereafter, executing an INSERT, DELETE or UPDATE on the view causes the associated triggers to fire. The real tables underlying the view are not modified (except possibly explicitly, by a trigger program). Example: Assuming that customer records are stored in the "customers" table, and that order records are stored in the "orders" table, the following trigger ensures that all associated orders are redirected when a customer changes his or her address: With this trigger installed, executing the statement: UPDATE customers SET address = '1 Main St.' WHERE name = 'Jack Jones'; causes the following to be automatically executed: UPDATE orders SET address = '1 Main St.' WHERE customer_name = 'Jack Jones'; Note that currently, triggers may behave oddly when created on tables with INTEGER PRIMARY KEY fields. If a BEFORE trigger program modifies the INTEGER PRIMARY KEY field of a row that will be subsequently updated by the statement that causes the trigger to fire, then the update may not occur. The workaround is to declare the table with a PRIMARY KEY column instead of an INTEGER PRIMARY KEY column. A special SQL function RAISE() may be used within a trigger-program, with the following syntax
When one of the first three forms is called during trigger-program execution, the specified ON CONFLICT processing is performed (either ABORT, FAIL or ROLLBACK) and the current query terminates. An error code of SQLITE_CONSTRAINT is returned to the user, along with the specified error message. When RAISE(IGNORE) is called, the remainder of the current trigger program, the statement that caused the trigger program to execute and any subsequent trigger programs that would of been executed are abandoned. No database changes are rolled back. If the statement that caused the trigger program to execute is itself part of a trigger program, then that trigger program resumes execution at the beginning of the next step. Triggers are removed using the DROP TRIGGER statement. CREATE VIEW
The CREATE VIEW command assigns a name to a pre-packaged SELECT statement. Once the view is created, it can be used in the FROM clause of another SELECT in place of a table name. If the "TEMP" or "TEMPORARY" keyword occurs in between "CREATE" and "VIEW" then the view that is created is only visible to the process that opened the database and is automatically deleted when the database is closed. If a <database-name> is specified, then the view is created in the named database. It is an error to specify both a <database-name> and the TEMP keyword, unless the <database-name> is "temp". If no database name is specified, and the TEMP keyword is not present, the table is created in the main database. You cannot COPY, DELETE, INSERT or UPDATE a view. Views are read-only in SQLite. However, in many cases you can use a TRIGGER on the view to accomplish the same thing. Views are removed with the DROP VIEW command. DELETE
The DELETE command is used to remove records from a table. The command consists of the "DELETE FROM" keywords followed by the name of the table from which records are to be removed. Without a WHERE clause, all rows of the table are removed. If a WHERE clause is supplied, then only those rows that match the expression are removed. DETACH DATABASE
This statement detaches an additional database connection previously attached using the ATTACH DATABASE statement. It is possible to have the same database file attached multiple times using different names, and detaching one connection to a file will leave the others intact. This statement will fail if SQLite is in the middle of a transaction. DROP INDEX
The DROP INDEX statement removes an index added with the CREATE INDEX statement. The index named is completely removed from the disk. The only way to recover the index is to reenter the appropriate CREATE INDEX command. The DROP INDEX statement does not reduce the size of the database file in the default mode. Empty space in the database is retained for later INSERTs. To remove free space in the database, use the VACUUM command. If AUTOVACUUM mode is enabled for a database then space will be freed automatically by DROP INDEX. DROP TABLE
The DROP TABLE statement removes a table added with the CREATE TABLE statement. The name specified is the table name. It is completely removed from the database schema and the disk file. The table can not be recovered. All indices associated with the table are also deleted. The DROP TABLE statement does not reduce the size of the database file in the default mode. Empty space in the database is retained for later INSERTs. To remove free space in the database, use the VACUUM command. If AUTOVACUUM mode is enabled for a database then space will be freed automatically by DROP TABLE. The optional IF EXISTS clause suppresses the error that would normally result if the table does not exist. DROP TRIGGER
The DROP TRIGGER statement removes a trigger created by the CREATE TRIGGER statement. The trigger is deleted from the database schema. Note that triggers are automatically dropped when the associated table is dropped. DROP VIEW
The DROP VIEW statement removes a view created by the CREATE VIEW statement. The name specified is the view name. It is removed from the database schema, but no actual data in the underlying base tables is modified. EXPLAIN
The EXPLAIN command modifier is a non-standard extension. The idea comes from a similar command found in PostgreSQL, but the operation is completely different. If the EXPLAIN keyword appears before any other SQLite SQL command then instead of actually executing the command, the SQLite library will report back the sequence of virtual machine instructions it would have used to execute the command had the EXPLAIN keyword not been present. For additional information about virtual machine instructions see the architecture description or the documentation on available opcodes for the virtual machine. expression
This section is different from the others. Most other sections of this document talks about a particular SQL command. This section does not talk about a standalone command but about "expressions" which are subcomponents of most other commands. SQLite understands the following binary operators, in order from highest to lowest precedence: || * / % + - << >> & | < <= > >= = == != <> IN AND OR Supported unary operators are these: - + ! ~ NOT Note that there are two variations of the equals and not equals operators. Equals can be either = or ==. The non-equals operator can be either != or <>. The || operator is "concatenate" - it joins together the two strings of its operands. The operator % outputs the remainder of its left operand modulo its right operand. The result of any binary operator is a numeric value, except for the || concatenation operator which gives a string result. A literal value is an integer number or a floating point number. Scientific notation is supported. The "." character is always used as the decimal point even if the locale setting specifies "," for this role - the use of "," for the decimal point would result in syntactic ambiguity. A string constant is formed by enclosing the string in single quotes ('). A single quote within the string can be encoded by putting two single quotes in a row - as in Pascal. C-style escapes using the backslash character are not supported because they are not standard SQL. BLOB literals are string literals containing hexadecimal data and preceded by a single "x" or "X" character. For example: X'53514697465' A literal value can also be the token "NULL". A parameter specifies a placeholder in the expression for a literal value that is filled in at runtime using the sqlite3_bind API. Parameters can take several forms:
Parameters that are not assigned values using sqlite3_bind are treated as NULL. The LIKE operator does a pattern matching comparison. The operand to the right contains the pattern, the left hand operand contains the string to match against the pattern. A percent symbol % in the pattern matches any sequence of zero or more characters in the string. An underscore _ in the pattern matches any single character in the string. Any other character matches itself or it's lower/upper case equivalent (i.e. case-insensitive matching). (A bug: SQLite only understands upper/lower case for 7-bit Latin characters. Hence the LIKE operator is case sensitive for 8-bit iso8859 characters or UTF-8 characters. For example, the expression 'a' LIKE 'A' is TRUE but 'æ' LIKE 'Æ' is FALSE.). If the optional ESCAPE clause is present, then the expression following the ESCAPE keyword must evaluate to a string consisting of a single character. This character may be used in the LIKE pattern to include literal percent or underscore characters. The escape character followed by a percent symbol, underscore or itself matches a literal percent symbol, underscore or escape character in the string, respectively. The infix LIKE operator is implemented by calling the user function like(X,Y). The LIKE operator is not case sensitive and will match upper case characters on one side against lower case characters on the other. (A bug: SQLite only understands upper/lower case for 7-bit Latin characters. Hence the LIKE operator is case sensitive for 8-bit iso8859 characters or UTF-8 characters. For example, the expression 'a' LIKE 'A' is TRUE but 'æ' LIKE 'Æ' is FALSE.).The infix LIKE operator is implemented by calling the user function like(X,Y). If an ESCAPE clause is present, it adds a third parameter to the function call. If the functionality of LIKE can be overridden by defining an alternative implementation of the like() SQL function. The GLOB operator is similar to LIKE but uses the Unix file globbing syntax for its wildcards. Also, GLOB is case sensitive, unlike LIKE. Both GLOB and LIKE may be preceded by the NOT keyword to invert the sense of the test. The infix GLOB operator is implemented by calling the user function glob(X,Y) and can be modified by overriding that function. The REGEXP operator is a special syntax for the regexp() user function. No regexp() user function is defined by default and so use of the REGEXP operator will normally result in an error message. If a user-defined function named "regexp" is defined at run-time, that function will be called in order to implement the REGEXP operator. A column name can be any of the names defined in the CREATE TABLE statement or one of the following special identifiers: "ROWID", "OID", or "_ROWID_". These special identifiers all describe the unique random integer key (the "row key") associated with every row of every table. The special identifiers only refer to the row key if the CREATE TABLE statement does not define a real column with the same name. Row keys act like read-only columns. A row key can be used anywhere a regular column can be used, except that you cannot change the value of a row key in an UPDATE or INSERT statement. "SELECT * ..." does not return the row key. SELECT statements can appear in expressions as either the right-hand operand of the IN operator, as a scalar quantity, or as the operand of an EXISTS operator. As a scalar quantity or the operand of an IN operator, the SELECT should have only a single column in its result. Compound SELECTs (connected with keywords like UNION or EXCEPT) are allowed. With the EXISTS operator, the columns in the result set of the SELECT are ignored and the expression returns TRUE if one or more rows exist and FALSE if the result set is empty. If no terms in the SELECT expression refer to value in the containing query, then the expression is evaluated once prior to any other processing and the result is reused as necessary. If the SELECT expression does contain variables from the outer query, then the SELECT is reevaluated every time it is needed. When a SELECT is the right operand of the IN operator, the IN operator returns TRUE if the result of the left operand is any of the values generated by the select. The IN operator may be preceded by the NOT keyword to invert the sense of the test. When a SELECT appears within an expression but is not the right operand of an IN operator, then the first row of the result of the SELECT becomes the value used in the expression. If the SELECT yields more than one result row, all rows after the first are ignored. If the SELECT yields no rows, then the value of the SELECT is NULL. A CAST expression changes the datatype of the Both simple and aggregate functions are supported. A simple function can be used in any expression. Simple functions return a result immediately based on their inputs. Aggregate functions may only be used in a SELECT statement. Aggregate functions compute their result across all rows of the result set. The functions shown below are available by default. Additional functions may be written in C and added to the database engine using the sqlite3_create_function() API.
The aggregate functions shown below are available by default. Additional aggregate functions written in C may be added using the sqlite3_create_function() API. In any aggregate function that takes a single argument, that argument can be preceeded by the keyword DISTINCT. In such cases, duplicate elements are filtered before being passed into the aggregate function. For example, the function "count(distinct X)" will return the number of distinct values of column X instead of the total number of non-null values in column X.
INSERT
The INSERT statement comes in two basic forms. The first form (with the "VALUES" keyword) creates a single new row in an existing table. If no column-list is specified then the number of values must be the same as the number of columns in the table. If a column-list is specified, then the number of values must match the number of specified columns. Columns of the table that do not appear in the column list are filled with the default value, or with NULL if not default value is specified. The second form of the INSERT statement takes it data from a SELECT statement. The number of columns in the result of the SELECT must exactly match the number of columns in the table if no column list is specified, or it must match the number of columns name in the column list. A new entry is made in the table for every row of the SELECT result. The SELECT may be simple or compound. If the SELECT statement has an ORDER BY clause, the ORDER BY is ignored. The optional conflict-clause allows the specification of an alternative constraint conflict resolution algorithm to use during this one command. See the section titled ON CONFLICT for additional information. For compatibility with MySQL, the parser allows the use of the single keyword REPLACE as an alias for "INSERT OR REPLACE". ON CONFLICT clause
The ON CONFLICT clause is not a separate SQL command. It is a non-standard clause that can appear in many other SQL commands. It is given its own section in this document because it is not part of standard SQL and therefore might not be familiar. The syntax for the ON CONFLICT clause is as shown above for the CREATE TABLE command. For the INSERT and UPDATE commands, the keywords "ON CONFLICT" are replaced by "OR", to make the syntax seem more natural. For example, instead of "INSERT ON CONFLICT IGNORE" we have "INSERT OR IGNORE". The keywords change but the meaning of the clause is the same either way. The ON CONFLICT clause specifies an algorithm used to resolve constraint conflicts. There are five choices: ROLLBACK, ABORT, FAIL, IGNORE, and REPLACE. The default algorithm is ABORT. This is what they mean:
The algorithm specified in the OR clause of a INSERT or UPDATE overrides any algorithm specified in a CREATE TABLE. If no algorithm is specified anywhere, the ABORT algorithm is used. REINDEX
The REINDEX command is used to delete and recreate indices from scratch. This is useful when the definition of a collation sequence has changed. In the first form, all indices in all attached databases that use the named collation sequence are recreated. In the second form, if [database-name.]table/index-name identifies a table, then all indices associated with the table are rebuilt. If an index is identified, then only this specific index is deleted and recreated. If no database-name is specified and there exists both a table or index and a collation sequence of the specified name, then indices associated with the collation sequence only are reconstructed. This ambiguity may be dispelled by always specifying a database-name when reindexing a specific table or index. REPLACE
The REPLACE command is an alias for the "INSERT OR REPLACE" variant of the INSERT command. This alias is provided for compatibility with MySQL. See the INSERT command documentation for additional information. SELECT
The SELECT statement is used to query the database. The result of a SELECT is zero or more rows of data where each row has a fixed number of columns. The number of columns in the result is specified by the expression list in between the SELECT and FROM keywords. Any arbitrary expression can be used as a result. If a result expression is * then all columns of all tables are substituted for that one expression. If the expression is the name of a table followed by .* then the result is all columns in that one table. The DISTINCT keyword causes a subset of result rows to be returned, in which each result row is different. NULL values are not treated as distinct from each other. The default behavior is that all result rows be returned, which can be made explicit with the keyword ALL. The query is executed against one or more tables specified after the FROM keyword. If multiple tables names are separated by commas, then the query is against the cross join of the various tables. The full SQL-92 join syntax can also be used to specify joins. A sub-query in parentheses may be substituted for any table name in the FROM clause. The entire FROM clause may be omitted, in which case the result is a single row consisting of the values of the expression list. The WHERE clause can be used to limit the number of rows over which the query operates. The GROUP BY clauses causes one or more rows of the result to be combined into a single row of output. This is especially useful when the result contains aggregate functions. The expressions in the GROUP BY clause do not have to be expressions that appear in the result. The HAVING clause is similar to WHERE except that HAVING applies after grouping has occurred. The HAVING expression may refer to values, even aggregate functions, that are not in the result. The ORDER BY clause causes the output rows to be sorted. The argument to ORDER BY is a list of expressions that are used as the key for the sort. The expressions do not have to be part of the result for a simple SELECT, but in a compound SELECT each sort expression must exactly match one of the result columns. Each sort expression may be optionally followed by a COLLATE keyword and the name of a collating function used for ordering text and/or keywords ASC or DESC to specify the sort order. The LIMIT clause places an upper bound on the number of rows returned in the result. A negative LIMIT indicates no upper bound. The optional OFFSET following LIMIT specifies how many rows to skip at the beginning of the result set. In a compound query, the LIMIT clause may only appear on the final SELECT statement. The limit is applied to the entire query not to the individual SELECT statement to which it is attached. Note that if the OFFSET keyword is used in the LIMIT clause, then the limit is the first number and the offset is the second number. If a comma is used instead of the OFFSET keyword, then the offset is the first number and the limit is the second number. This seeming contradition is intentional - it maximizes compatibility with legacy SQL database systems. A compound SELECT is formed from two or more simple SELECTs connected by one of the operators UNION, UNION ALL, INTERSECT, or EXCEPT. In a compound SELECT, all the constituent SELECTs must specify the same number of result columns. There may be only a single ORDER BY clause at the end of the compound SELECT. The UNION and UNION ALL operators combine the results of the SELECTs to the right and left into a single big table. The difference is that in UNION all result rows are distinct where in UNION ALL there may be duplicates. The INTERSECT operator takes the intersection of the results of the left and right SELECTs. EXCEPT takes the result of left SELECT after removing the results of the right SELECT. When three or more SELECTs are connected into a compound, they group from left to right. UPDATE
The UPDATE statement is used to change the value of columns in selected rows of a table. Each assignment in an UPDATE specifies a column name to the left of the equals sign and an arbitrary expression to the right. The expressions may use the values of other columns. All expressions are evaluated before any assignments are made. A WHERE clause can be used to restrict which rows are updated. The optional conflict-clause allows the specification of an alternative constraint conflict resolution algorithm to use during this one command. See the section titled ON CONFLICT for additional information. VACUUM
The VACUUM command is an SQLite extension modeled after a similar command found in PostgreSQL. If VACUUM is invoked with the name of a table or index then it is suppose to clean up the named table or index. In version 1.0 of SQLite, the VACUUM command would invoke gdbm_reorganize() to clean up the backend database file. VACUUM became a no-op when the GDBM backend was removed from SQLITE in version 2.0.0. VACUUM was reimplemented in version 2.8.1. The index or table name argument is now ignored. When an object (table, index, or trigger) is dropped from the database, it leaves behind empty space. This makes the database file larger than it needs to be, but can speed up inserts. In time inserts and deletes can leave the database file structure fragmented, which slows down disk access to the database contents. The VACUUM command cleans the main database by copying its contents to a temporary database file and reloading the original database file from the copy. This eliminates free pages, aligns table data to be contiguous, and otherwise cleans up the database file structure. It is not possible to perform the same process on an attached database file. This command will fail if there is an active transaction. This command has no effect on an in-memory database. As of SQLite version 3.1, an alternative to using the VACUUM command is auto-vacuum mode, enabled using the auto_vacuum pragma. PRAGMA command syntax
The pragmas that take an integer value also accept symbolic names. The strings "on", "true", and "yes" are equivalent to 1. The strings "off", "false", and "no" are equivalent to 0. These strings are case- insensitive, and do not require quotes. An unrecognized string will be treated as 1, and will not generate an error. When the value is returned it is as an integer. The PRAGMA command is a special command used to modify the operation of the SQLite library or to query the library for internal (non-table) data. The PRAGMA command is issued using the same interface as other SQLite commands (e.g. SELECT, INSERT) but is different in the following important respects:
The available pragmas fall into four basic categories:
Pragmas to modify library operation
Pragmas to query the database schema
Pragmas to query/modify version values
Pragmas to debug the library
SQLite keywordsThe following keywords are used by SQLite. Most are either reserved words in SQL-92 or were listed as potential reserved words. Those which aren't are shown in italics. Not all of these words are actually used by SQLite. Keywords are not reserved in SQLite. Any keyword can be used as an identifier for SQLite objects (columns, databases, indexes, tables, triggers, views, ...) but must generally be enclosed by brackets or quotes to avoid confusing the parser. Keyword matching in SQLite is case-insensitive. Keywords can be used as identifiers in three ways:
Fallback KeywordsThese keywords can be used as identifiers for SQLite objects without delimiters. ABORT AFTER ASC ATTACH BEFORE BEGIN DEFERRED CASCADE CLUSTER CONFLICT COPY CROSS DATABASE DELIMITERS DESC DETACH EACH END EXPLAIN FAIL FOR FULL IGNORE IMMEDIATE INITIALLY INNER INSTEAD KEY LEFT MATCH NATURAL OF OFFSET OUTER PRAGMA RAISE REPLACE RESTRICT RIGHT ROW STATEMENT TEMP TEMPORARY TRIGGER VACUUM VIEW Normal keywordsThese keywords can be used as identifiers for SQLite objects, but must be enclosed in brackets or quotes for SQLite to recognize them as an identifier. ALL AND AS BETWEEN BY CASE CHECK COLLATE COMMIT CONSTRAINT CREATE DEFAULT DEFERRABLE DELETE DISTINCT DROP ELSE EXCEPT FOREIGN FROM GLOB GROUP HAVING IN INDEX INSERT INTERSECT INTO IS ISNULL JOIN LIKE LIMIT NOT NOTNULL NULL ON OR ORDER PRIMARY REFERENCES ROLLBACK SELECT SET TABLE THEN TRANSACTION UNION UNIQUE UPDATE USING VALUES WHEN WHERE Special wordsThe following are not keywords in SQLite, but are used as names of system objects. They can be used as an identifier for a different type of object. _ROWID_ MAIN OID ROWID SQLITE_MASTER SQLITE_TEMP_MASTER
Datatypes In SQLite Version 31. Storage ClassesVersion 2 of SQLite stores all column values as ASCII text. Version 3 enhances this by providing the ability to store integer and real numbers in a more compact format and the capability to store BLOB data. Each value stored in an SQLite database (or manipulated by the database engine) has one of the following storage classes:
As in SQLite version 2, any column in a version 3 database except an INTEGER PRIMARY KEY may be used to store any type of value. The exception to this rule is described below under 'Strict Affinity Mode'. All values supplied to SQLite, whether as literals embedded in SQL statements or values bound to pre-compiled SQL statements are assigned a storage class before the SQL statement is executed. Under circumstances described below, the database engine may convert values between numeric storage classes (INTEGER and REAL) and TEXT during query execution. Storage classes are initially assigned as follows:
The storage class of a value that is the result of an SQL scalar operator depends on the outermost operator of the expression. User-defined functions may return values with any storage class. It is not generally possible to determine the storage class of the result of an expression at compile time. 2. Column AffinityIn SQLite version 3, the type of a value is associated with the value itself, not with the column or variable in which the value is stored. (This is sometimes called manifest typing.) All other SQL databases engines that we are aware of use the more restrictive system of static typing where the type is associated with the container, not the value. In order to maximize compatibility between SQLite and other database engines, SQLite support the concept of "type affinity" on columns. The type affinity of a column is the recommended type for data stored in that column. The key here is that the type is recommended, not required. Any column can still store any type of data, in theory. It is just that some columns, given the choice, will prefer to use one storage class over another. The preferred storage class for a column is called its "affinity". Each column in an SQLite 3 database is assigned one of the following type affinities:
A column with TEXT affinity stores all data using storage classes NULL, TEXT or BLOB. If numerical data is inserted into a column with TEXT affinity it is converted to text form before being stored. A column with NUMERIC affinity may contain values using all five storage classes. When text data is inserted into a NUMERIC column, an attempt is made to convert it to an integer or real number before it is stored. If the conversion is successful, then the value is stored using the INTEGER or REAL storage class. If the conversion cannot be performed the value is stored using the TEXT storage class. No attempt is made to convert NULL or blob values. A column that uses INTEGER affinity behaves in the same way as a column with NUMERIC affinity, except that if a real value with no floating point component (or text value that converts to such) is inserted it is converted to an integer and stored using the INTEGER storage class. A column with REAL affinity behaves like a column with NUMERIC affinity except that it forces integer values into floating point representation. (As an optimization, integer values are stored on disk as integers in order to take up less space and are only converted to floating point as the value is read out of the table.) A column with affinity NONE does not prefer one storage class over another. It makes no attempt to coerce data before it is inserted. 2.1 Determination Of Column AffinityThe type affinity of a column is determined by the declared type of the column, according to the following rules:
If a table is created using a "CREATE TABLE <table> AS SELECT..." statement, then all columns have no datatype specified and they are given no affinity. 2.2 Column Affinity Example3. Comparison ExpressionsLike SQLite version 2, version 3 features the binary comparison operators '=', '<', '<=', '>=' and '!=', an operation to test for set membership, 'IN', and the ternary comparison operator 'BETWEEN'. The results of a comparison depend on the storage classes of the two values being compared, according to the following rules:
SQLite may attempt to convert values between the numeric storage classes (INTEGER and REAL) and TEXT before performing a comparison. For binary comparisons, this is done in the cases enumerated below. The term "expression" used in the bullet points below means any SQL scalar expression or literal other than a column value.
In SQLite, the expression "a BETWEEN b AND c" is equivalent to "a >= b AND a <= c", even if this means that different affinities are applied to 'a' in each of the comparisons required to evaluate the expression. Expressions of the type "a IN (SELECT b ....)" are handled by the three rules enumerated above for binary comparisons (e.g. in a similar manner to "a = b"). For example if 'b' is a column value and 'a' is an expression, then the affinity of 'b' is applied to 'a' before any comparisons take place. SQLite treats the expression "a IN (x, y, z)" as equivalent to "a = z OR a = y OR a = z". 3.1 Comparison Example4. OperatorsAll mathematical operators (which is to say, all operators other than the concatenation operator "||") apply NUMERIC affinity to all operands prior to being carried out. If one or both operands cannot be converted to NUMERIC then the result of the operation is NULL. For the concatenation operator, TEXT affinity is applied to both operands. If either operand cannot be converted to TEXT (because it is NULL or a BLOB) then the result of the concatenation is NULL. 5. Sorting, Grouping and Compound SELECTsWhen values are sorted by an ORDER by clause, values with storage class NULL come first, followed by INTEGER and REAL values interspersed in numeric order, followed by TEXT values usually in memcmp() order, and finally BLOB values in memcmp() order. No storage class conversions occur before the sort. When grouping values with the GROUP BY clause values with different storage classes are considered distinct, except for INTEGER and REAL values which are considered equal if they are numerically equal. No affinities are applied to any values as the result of a GROUP by clause. The compound SELECT operators UNION, INTERSECT and EXCEPT perform implicit comparisons between values. Before these comparisons are performed an affinity may be applied to each value. The same affinity, if any, is applied to all values that may be returned in a single column of the compound SELECT result set. The affinity applied is the affinity of the column returned by the left most component SELECTs that has a column value (and not some other kind of expression) in that position. If for a given compound SELECT column none of the component SELECTs return a column value, no affinity is applied to the values from that column before they are compared. 6. Other Affinity ModesThe above sections describe the operation of the database engine in 'normal' affinity mode. SQLite version 3 will feature two other affinity modes, as follows:
7. User-defined Collation SequencesBy default, when SQLite compares two text values, the result of the comparison is determined using memcmp(), regardless of the encoding of the string. SQLite v3 provides the ability for users to supply arbitrary comparison functions, known as user-defined collation sequences, to be used instead of memcmp(). Aside from the default collation sequence BINARY, implemented using memcmp(), SQLite features one extra built-in collation sequences intended for testing purposes, the NOCASE collation:
7.1 Assigning Collation Sequences from SQLEach column of each table has a default collation type. If a collation type other than BINARY is required, a COLLATE clause is specified as part of the column definition to define it. Whenever two text values are compared by SQLite, a collation sequence is used to determine the results of the comparison according to the following rules. Sections 3 and 5 of this document describe the circumstances under which such a comparison takes place. For binary comparison operators (=, <, >, <= and >=) if either operand is a column, then the default collation type of the column determines the collation sequence to use for the comparison. If both operands are columns, then the collation type for the left operand determines the collation sequence used. If neither operand is a column, then the BINARY collation sequence is used. The expression "x BETWEEN y and z" is equivalent to "x >= y AND x <= z". The expression "x IN (SELECT y ...)" is handled in the same way as the expression "x = y" for the purposes of determining the collation sequence to use. The collation sequence used for expressions of the form "x IN (y, z ...)" is the default collation type of x if x is a column, or BINARY otherwise. An ORDER BY clause that is part of a SELECT statement may be assigned a collation sequence to be used for the sort operation explicitly. In this case the explicit collation sequence is always used. Otherwise, if the expression sorted by an ORDER BY clause is a column, then the default collation type of the column is used to determine sort order. If the expression is not a column, then the BINARY collation sequence is used. 7.2 Collation Sequences ExampleThe examples below identify the collation sequences that would be used to determine the results of text comparisons that may be performed by various SQL statements. Note that a text comparison may not be required, and no collation sequence used, in the case of numeric, blob or NULL values. Distinctive Features Of SQLiteThis page highlights some of the characteristics of SQLite that are unusual and which make SQLite different from many other SQL database engines. Zero-Configuration SQLite does not need to be "installed" before it is used. There is no "setup" procedure. There is no server process that needs to be started, stopped, or configured. There is no need for an administrator to create a new database instance or assign access permissions to users. SQLite uses no configuration files. Nothing needs to be done to tell the system that SQLite is running. No actions are required to recover after a system crash or power failure. There is nothing to troubleshoot. Serverless Most SQL database engines are implemented as a separate server process. Programs that want to access the database communicate with the server using some kind of interprocess communcation (typically TCP/IP) to send requests to the server and to receive back results. SQLite does not work this way. With SQLite, the process that wants to access the database reads and writes directly from the database files on disk. There is no intermediary server process. Single Database File An SQLite database is a single ordinary disk file that can be located anywhere in the directory hierarchy. If SQLite can read the disk file then it can read anything in the database. If the disk file and its directory are writable, then SQLite can change anything in the database. Database files can easily be copied onto a USB memory stick or emailed for sharing. Compact When optimized for size, the whole SQLite library with everything enabled is less than 225KiB in size (as measured on an ix86 using the "size" utility from the GNU compiler suite.) Unneeded features can be disabled at compile-time to further reduce the size of the library to under 170KiB if desired. Manifest typing Most SQL database engines use static typing. A datatype is associated with each column in a table and only values of that particular datatype are allowed to be stored in that column. SQLite relaxes this restriction by using manifest typing. In manifest typing, the datatype is a property of the value itself, not of the column in which the value is stored. SQLite thus allows the user to store any value of any datatype into any column regardless of the declared type of that column. (There are some exceptions to this rule: An INTEGER PRIMARY KEY column may only store integers. And SQLite attempts to coerce values into the declared datatype of the column when it can.) Variable-length records Most other SQL database engines allocated a fixed amount of disk space for each row in most tables. They play special tricks for handling BLOBs and CLOBs which can be of wildly varying length. But for most tables, if you declare a column to be a VARCHAR(100) then the database engine will allocate 100 bytes of disk space regardless of how much information you actually store in that column. Readable source code The source code to SQLite is designed to be readable and accessible to the average programmer. All procedures and data structures and many automatic variables are carefully commented with useful information about what they do. Boilerplate commenting is omitted. SQL statements compile into virtual machine code Every SQL database engine compiles each SQL statement into some kind of internal data structure which is then used to carry out the work of the statement. But in most SQL engines that internal data structure is a complex web of interlinked structures and objects. In SQLite, the compiled form of statements is a short program in a machine-language like representation. Users of the database can view this virtual machine language by prepending the EXPLAIN keyword to a query. Public domain The source code for SQLite is in the public domain. No claim of copyright is made on any part of the core source code. (The documentation and test code is a different matter - some sections of documentation and test logic are governed by open-sources licenses.) All contributors to the SQLite core software have signed affidavits specifically disavowing any copyright interest in the code. This means that anybody is able to legally do anything they want with the SQLite source code. SQL language extensions SQLite provides a number of enhancements to the SQL language not normally found in other database engines. The EXPLAIN keyword and manifest typing have already been mentioned above. SQLite also provides statements such as REPLACE and the ON CONFLICT clause that allow for added control over the resolution of constraint conflicts. SQLite supports ATTACH and DETACH commands that allow multiple independent databases to be used together in the same query. And SQLite defines APIs that allows the user to add new SQL functions and collating sequences. NULL Handling in SQLite Versus Other Database EnginesThe goal is to make SQLite handle NULLs in a standards-compliant way. But the descriptions in the SQL standards on how to handle NULLs seem ambiguous. It is not clear from the standards documents exactly how NULLs should be handled in all circumstances. So instead of going by the standards documents, various popular SQL engines were tested to see how they handle NULLs. The idea was to make SQLite work like all the other engines. A SQL test script was developed and run by volunteers on various SQL RDBMSes and the results of those tests were used to deduce how each engine processed NULL values. The original tests were run in May of 2002. A copy of the test script is found at the end of this document. SQLite was originally coded in such a way that the answer to all questions in the chart below would be "Yes". But the experiments run on other SQL engines showed that none of them worked this way. So SQLite was modified to work the same as Oracle, PostgreSQL, and DB2. This involved making NULLs indistinct for the purposes of the SELECT DISTINCT statement and for the UNION operator in a SELECT. NULLs are still distinct in a UNIQUE column. This seems somewhat arbitrary, but the desire to be compatible with other engines outweighted that objection. It is possible to make SQLite treat NULLs as distinct for the purposes of the SELECT DISTINCT and UNION. To do so, one should change the value of the NULL_ALWAYS_DISTINCT #define in the sqliteInt.h source file and recompile.
The following table shows the results of the NULL handling experiments.
The following script was used to gather information for the table above. -- I have about decided that SQL's treatment of NULLs is capricious and cannot be -- deduced by logic. It must be discovered by experiment. To that end, I have -- prepared the following script to test how various SQL databases deal with NULL. -- My aim is to use the information gather from this script to make SQLite as much -- like other databases as possible. -- -- If you could please run this script in your database engine and mail the results -- to me at drh@hwaci.com, that will be a big help. Please be sure to identify the -- database engine you use for this test. Thanks. -- -- If you have to change anything to get this script to run with your database -- engine, please send your revised script together with your results. -- -- Create a test table with data create table t1(a int, b int, c int); insert into t1 values(1,0,0); insert into t1 values(2,0,1); insert into t1 values(3,1,0); insert into t1 values(4,1,1); insert into t1 values(5,null,0); insert into t1 values(6,null,1); insert into t1 values(7,null,null); -- Check to see what CASE does with NULLs in its test expressions select a, case when b<>0 then 1 else 0 end from t1; select a+10, case when not b<>0 then 1 else 0 end from t1; select a+20, case when b<>0 and c<>0 then 1 else 0 end from t1; select a+30, case when not (b<>0 and c<>0) then 1 else 0 end from t1; select a+40, case when b<>0 or c<>0 then 1 else 0 end from t1; select a+50, case when not (b<>0 or c<>0) then 1 else 0 end from t1; select a+60, case b when c then 1 else 0 end from t1; select a+70, case c when b then 1 else 0 end from t1; -- What happens when you multiple a NULL by zero? select a+80, b*0 from t1; select a+90, b*c from t1; -- What happens to NULL for other operators? select a+100, b+c from t1; -- Test the treatment of aggregate operators select count(*), count(b), sum(b), avg(b), min(b), max(b) from t1; -- Check the behavior of NULLs in WHERE clauses select a+110 from t1 where b<10; select a+120 from t1 where not b>10; select a+130 from t1 where b<10 OR c=1; select a+140 from t1 where b<10 AND c=1; select a+150 from t1 where not (b<10 AND c=1); select a+160 from t1 where not (c=1 AND b<10); -- Check the behavior of NULLs in a DISTINCT query select distinct b from t1; -- Check the behavior of NULLs in a UNION query select b from t1 union select b from t1; -- Create a new table with a unique column. Check to see if NULLs are considered -- to be distinct. create table t2(a int, b int unique); insert into t2 values(1,1); insert into t2 values(2,null); insert into t2 values(3,null); select * from t2; drop table t1; drop table t2; SQL Features That SQLite Does Not ImplementRather than try to list all the features of SQL92 that SQLite does support, it is much easier to list those that it does not. Unsupported features of SQL92 are shown below. The order of this list gives some hint as to when a feature might be added to SQLite. Those features near the top of the list are likely to be added in the near future. There are no immediate plans to add features near the bottom of the list.
If you find other SQL92 features that SQLite does not support, please add them to the Wiki page at http://www.sqlite.org/cvstrac/wiki?p=UnsupportedSql SQLite AutoincrementIn SQLite, every row of every table has an integer ROWID. The ROWID for each row is unique among all rows in the same table. In SQLite version 2.8 the ROWID is a 32-bit signed integer. Version 3.0 of SQLite expanded the ROWID to be a 64-bit signed integer. You can access the ROWID of an SQLite table using one the special column names ROWID, _ROWID_, or OID. Except if you declare an ordinary table column to use one of those special names, then the use of that name will refer to the declared column not to the internal ROWID. If a table contains a column of type INTEGER PRIMARY KEY, then that column becomes an alias for the ROWID. You can then access the ROWID using any of four different names, the original three names described above or the name given to the INTEGER PRIMARY KEY column. All these names are aliases for one another and work equally well in any context. When a new row is inserted into an SQLite table, the ROWID can either be specified as part of the INSERT statement or it can be assigned automatically by the database engine. To specify a ROWID manually, just include it in the list of values to be inserted. For example: CREATE TABLE test1(a INT, b TEXT); INSERT INTO test1(rowid, a, b) VALUES(123, 5, 'hello'); If no ROWID is specified on the insert, an appropriate ROWID is created automatically. The usual algorithm is to give the newly created row a ROWID that is one larger than the largest ROWID in the table prior to the insert. If the table is initially empty, then a ROWID of 1 is used. If the largest ROWID is equal to the largest possible integer (9223372036854775807 in SQLite version 3.0 and later) then the database engine starts picking candidate ROWIDs at random until it finds one that is not previously used. The normal ROWID selection algorithm described above will generate monotonically increasing unique ROWIDs as long as you never use the maximum ROWID value and you never delete the entry in the table with the largest ROWID. If you ever delete rows or if you ever create a row with the maximum possible ROWID, then ROWIDs from previously deleted rows might be reused when creating new rows and newly created ROWIDs might not be in strictly accending order. The AUTOINCREMENT KeywordIf a column has the type INTEGER PRIMARY KEY AUTOINCREMENT then a slightly different ROWID selection algorithm is used. The ROWID chosen for the new row is one larger than the largest ROWID that has ever before existed in that same table. If the table has never before contained any data, then a ROWID of 1 is used. If the table has previously held a row with the largest possible ROWID, then new INSERTs are not allowed and any attempt to insert a new row will fail with an SQLITE_FULL error. SQLite keeps track of the largest ROWID that a table has ever held using the special SQLITE_SEQUENCE table. The SQLITE_SEQUENCE table is created and initialized automatically whenever a normal table that contains an AUTOINCREMENT column is created. The content of the SQLITE_SEQUENCE table can be modified using ordinary UPDATE, INSERT, and DELETE statements. But making modifications to this table will likely perturb the AUTOINCREMENT key generation algorithm. Make sure you know what you are doing before you undertake such changes. The behavior implemented by the AUTOINCREMENT keyword is subtly different from the default behavior. With AUTOINCREMENT, rows with automatically selected ROWIDs are guaranteed to have ROWIDs that have never been used before by the same table in the same database. And the automatically generated ROWIDs are guaranteed to be monotonically increasing. These are important properties in certain applications. But if your application does not need these properties, you should probably stay with the default behavior since the use of AUTOINCREMENT requires additional work to be done as each row is inserted and thus causes INSERTs to run a little slower. Back Home |
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