FOR UPDATE

Lock rows for write operations. This clause prevents other transactions from modifying or reading the selected rows until the current transaction ends.

InnoDB supports row-level locking. Selected rows can be locked using LOCK IN SHARE MODE or FOR UPDATE. In both cases, a lock is acquired on the rows read by the query, and it will be released when the current transaction is committed.

The FOR UPDATE clause of SELECT applies only when autocommit is set to 0 or the SELECT is enclosed in a transaction. A lock is acquired on the rows, and other transactions are prevented from writing the rows, acquire locks, and from reading them (unless their isolation level is READ UNCOMMITTED).

If autocommit is set to 1, the LOCK IN SHARE MODE and FOR UPDATE clauses have no effect in InnoDB. For non-transactional storage engines like MyISAM and ARIA, a table level lock will be taken even if autocommit is set to 1.

If the isolation level is set to SERIALIZABLE, all plain SELECT statements are converted to SELECT ... LOCK IN SHARE MODE.

Example

SELECT * FROM trans WHERE period=2001 FOR UPDATE;

See Also

• SELECT

• LOCK IN SHARE MODE

• InnoDB Lock Modes

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