# FOR UPDATE

InnoDB supports row-level locking. Selected rows can be locked using [LOCK IN SHARE MODE](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/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](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/select) applies only when [autocommit](https://mariadb.com/docs/server/server-management/variables-and-modes/server-system-variables#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](https://mariadb.com/docs/server/reference/sql-statements/administrative-sql-statements/set-commands/set-transaction)).

If `autocommit` is set to 1, the [LOCK IN SHARE MODE](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/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](https://mariadb.com/docs/server/reference/sql-statements/administrative-sql-statements/set-commands/set-transaction), all plain `SELECT` statements are converted to `SELECT ... LOCK IN SHARE MODE`.

## Example

```sql
SELECT * FROM trans WHERE period=2001 FOR UPDATE;
```

## See Also

* [SELECT](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/select)
* [LOCK IN SHARE MODE](https://mariadb.com/docs/server/reference/sql-statements/data-manipulation/selecting-data/lock-in-share-mode)
* [InnoDB Lock Modes](https://mariadb.com/docs/server/server-usage/storage-engines/innodb/innodb-lock-modes)

<sub>*This page is licensed: CC BY-SA / Gnu FDL*</sub>

{% @marketo/form formId="4316" %}