6.4.6 DELETE Syntax

DELETE [LOW_PRIORITY] [QUICK] FROM table_name
       [WHERE where_definition]
       [ORDER BY ...]
       [LIMIT row_count]

or

DELETE [LOW_PRIORITY] [QUICK] table_name[.*] [, table_name[.*] ...]
       FROM table-references
       [WHERE where_definition]

or

DELETE [LOW_PRIORITY] [QUICK]
       FROM table_name[.*] [, table_name[.*] ...]
       USING table-references
       [WHERE where_definition]

DELETE deletes rows from table_name that satisfy the condition given by where_definition, and returns the number of records deleted.

If you issue a DELETE with no WHERE clause, all rows are deleted. If you do this in AUTOCOMMIT mode, this works as TRUNCATE. See section 6.4.7 TRUNCATE Syntax. In MySQL 3.23, DELETE without a WHERE clause will return zero as the number of affected records.

If you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement of this form:

mysql> DELETE FROM table_name WHERE 1>0;

Note that this is much slower than DELETE FROM table_name with no WHERE clause, because it deletes rows one at a time.

If you specify the keyword LOW_PRIORITY, execution of the DELETE is delayed until no other clients are reading from the table.

For MyISAM tables, if you specify the word QUICK then the storage engine will not merge index leaves during delete, which may speed up certain kind of deletes.

The speed of delete operations may also be affected by factors discussed in section 5.2.12 Speed of DELETE Queries.

In MyISAM tables, deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file-sizes, use the OPTIMIZE TABLE statement or the myisamchk utility to reorganise tables. OPTIMIZE TABLE is easier, but myisamchk is faster. See section 4.5.1 OPTIMIZE TABLE Syntax and section 4.4.6.10 Table Optimisation.

The first multi-table delete format is supported starting from MySQL 4.0.0. The second multi-table delete format is supported starting from MySQL 4.0.2.

The idea is that only matching rows from the tables listed before the FROM or before the USING clause are deleted. The effect is that you can delete rows from many tables at the same time and also have additional tables that are used for searching.

The .* after the table names is there just to be compatible with Access:

DELETE t1,t2 FROM t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id

or

DELETE FROM t1,t2 USING t1,t2,t3 WHERE t1.id=t2.id AND t2.id=t3.id

In the above case we delete matching rows just from tables t1 and t2.

If an ORDER BY clause is used (available from MySQL 4.0.0), the rows will be deleted in that order. This is really only useful in conjunction with LIMIT. For example:

DELETE FROM somelog
WHERE user = 'jcole'
ORDER BY timestamp
LIMIT 1

This will delete the oldest entry (by timestamp) where the row matches the WHERE clause.

The MySQL-specific LIMIT row_count option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE command doesn't take too much time. You can simply repeat the DELETE command until the number of affected rows is less than the LIMIT value.

From MySQL 4.0, you can specify multiple tables in the DELETE statement to delete rows from one or more tables depending on a particular condition in multiple tables. However, you can not use ORDER BY or LIMIT in a multi-table DELETE.

User Comments

In these section you don't say nothing about the parent - child restrictions (on InnoDB tables), because you cannot delete a parent row only. You have to delete the parent, and child row

When you delete a large portion of a MyISAM table, it may be necessary to run an optimize table job afterwards. When the data file becomes fragmented, or when the key distribution data comes out of sync with reality, then you will see large performance drops. Also, disk space will not be freed until you've run optimize.

These are not really bugs, but limitations that you should be aware of when you start deleting large amounts of data from your tables.

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