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SQL As Understood By SQLite

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The 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 percisely what parts of the SQL language SQLite does and does not support.

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.

CREATE TABLE

The basic structure of a CREATE TABLE statement is as follows:

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} } Syntax {sql-command} { CREATE TABLE ( [, ]* [, ]* ) } {column-def} { []* } {type} { | ( ) | ( , ) } {column-constraint} { NOT NULL | PRIMARY KEY [] | UNIQUE | CHECK ( ) } {constraint} { PRIMARY KEY ( [, ]* ) | UNIQUE ( [, ]* ) | CHECK ( ) } puts {

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. The only reserved table name is "sqlite_master" which is the name of the table that records the database schema.

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 is ignored. All information is stored as null-terminated strings. The constraints are also ignored, except that the PRIMARY KEY constraint will cause an index to be automatically created that implements the primary key. The name of the primary key index will be the table name with "__primary_key" appended. The index used for a primary key does not show up in the sqlite_master table, but a GDBM file is created for that index.

There are no arbitrary limits on the size of columns, on the number of columns, or on the number of constraints in a table.

The exact text of each CREATE TABLE statement is stored in the sqlite_master table. Everytime 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.

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CREATE INDEX

} Syntax {sql-statement} { CREATE INDEX ON ( [, ]* ) } {column-name} { [ ASC | DESC ] } puts {

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 since GDBM does not implement ordered keys, these keywords are ignored.

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.

The exact text of each CREATE INDEX statement is stored in the sqlite_master table. Everytime 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.

DROP TABLE

} Syntax {sql-command} { DROP TABLE } puts {

The DROP TABLE statement consists of the keywords "DROP TABLE" followed by the name of the table. The table named is completely removed from the disk. The table can not be recovered. All indices associated with the table are also reversibly deleted.

DROP INDEX

} Syntax {sql-command} { DROP INDEX } puts {

The DROP INDEX statement consists of the keywords "DROP INDEX" followed by the name of the index. The index named is completely removed from the disk. The only way to recover the index is to reenter the appropriate CREATE INDEX command.

VACUUM

} Syntax {sql-statement} { VACUUM [] } puts {

The VACUUM command is an SQLite extension modelled after a similar command found in PostgreSQL. If VACUUM is invoked with the name of a table or index, then the gdbm_reorganize() function is called on the corresponding GDBM file. If VACUUM is invoked with no arguments, then gdbm_reorganize() is call on every GDBM file in the database.

It is a good idea to run VACUUM after creating large indices, especially indices where a single index value refers to many entries in the data table. Reorganizing these indices will make the underlying GDBM file much smaller and will help queries to run much faster.

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