*** DRAFT ***

SQL As Understood By SQLite

[Top]

DELETE

delete-stmt:

syntax diagram delete-stmt

expr:

qualified-table-name:

with-clause:

The DELETE command removes records from the table identified by the qualified-table-name.

If the WHERE clause is not present, all records in the table are deleted. If a WHERE clause is supplied, then only those rows for which the result of evaluating the WHERE clause as a boolean expression is true are deleted.

Restrictions on DELETE Statements Within CREATE TRIGGER

The following restrictions apply to DELETE statements that occur within the body of a CREATE TRIGGER statement:

Optional LIMIT and ORDER BY clauses

If SQLite is compiled with the SQLITE_ENABLE_UPDATE_DELETE_LIMIT compile-time option, then the syntax of the DELETE statement is extended by the addition of optional ORDER BY and LIMIT clauses:

delete-stmt-limited:

syntax diagram delete-stmt-limited

If a DELETE statement has a LIMIT clause, the maximum number of rows that will be deleted is found by evaluating the accompanying expression and casting it to an integer value. If the result of the evaluating the LIMIT clause cannot be losslessly converted to an integer value, it is an error. A negative LIMIT value is interpreted as "no limit". If the DELETE statement also has an OFFSET clause, then it is similarly evaluated and cast to an integer value. Again, it is an error if the value cannot be losslessly converted to an integer. If there is no OFFSET clause, or the calculated integer value is negative, the effective OFFSET value is zero.

If the DELETE statement has an ORDER BY clause, then all rows that would be deleted in the absence of the LIMIT clause are sorted according to the ORDER BY. The first M rows, where M is the value found by evaluating the OFFSET clause expression, are skipped, and the following N, where N is the value of the LIMIT expression, are deleted. If there are less than N rows remaining after taking the OFFSET clause into account, or if the LIMIT clause evaluated to a negative value, then all remaining rows are deleted.

If the DELETE statement has no ORDER BY clause, then all rows that would be deleted in the absence of the LIMIT clause are assembled in an arbitrary order before applying the LIMIT and OFFSET clauses to determine the subset that are actually deleted.

The ORDER BY clause on a DELETE statement is used only to determine which rows fall within the LIMIT. The order in which rows are deleted is arbitrary and is not influenced by the ORDER BY clause.

The Truncate Optimization

When the WHERE is omitted from a DELETE statement and the table being deleted has no triggers, SQLite uses an optimization to erase the entire table content without having to visit each row of the table individually. This "truncate" optimization makes the delete run much faster. Prior to SQLite version 3.6.5, the truncate optimization also meant that the sqlite3_changes() and sqlite3_total_changes() interfaces and the count_changes pragma will not actually return the number of deleted rows. That problem has been fixed as of version 3.6.5.

The truncate optimization can be permanently disabled for all queries by recompiling SQLite with the SQLITE_OMIT_TRUNCATE_OPTIMIZATION compile-time switch.

The truncate optimization can also be disabled at runtime using the sqlite3_set_authorizer() interface. If an authorizer callback returns SQLITE_IGNORE for an SQLITE_DELETE action code, then the DELETE operation will proceed but the truncate optimization will be bypassed and rows will be deleted one by one.

*** DRAFT ***