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By default, MySQL runs in autocommit mode. This means that as soon as you execute an update, MySQL will store the update on disk.

If you are using transactions safe tables (like InnoDB, BDB, you can put MySQL into non-autocommit mode with the following command:

SET AUTOCOMMIT=0

After this you must use COMMIT to store your changes to disk or ROLLBACK if you want to ignore the changes you have made since the beginning of your transaction.

If you want to switch from AUTOCOMMIT mode for one series of statements, you can use the BEGIN or BEGIN WORK statement:

BEGIN;
SELECT @A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summmary=@A WHERE type=1;
COMMIT;

Note that if you are using non-transaction-safe tables, the changes will be stored at once, independent of the status of the autocommit mode.

If you do a ROLLBACK when you have updated a non-transactional table you will get an error (ER_WARNING_NOT_COMPLETE_ROLLBACK) as a warning. All transactional safe tables will be restored but any non-transactional table will not change.

If you are using BEGIN or SET AUTOCOMMIT=0, you should use the MySQL binary log for backups instead of the older update log. Transactions are stored in the binary log in one chunk, upon COMMIT, to ensure that transactions which are rolled back are not stored. See Binary log.

The following commands automatically end a transaction (as if you had done a COMMIT before executing the command):

Command	Command	Command
ALTER TABLE	BEGIN	CREATE INDEX
DROP DATABASE	DROP TABLE	RENAME TABLE
TRUNCATE

You can change the isolation level for transactions with SET TRANSACTION ISOLATION LEVEL .... See SET TRANSACTION.