Defining a Transaction
The default behavior of MySQL is to perform a COMMIT after the execution of each individual SQL statement, effectively turning every statement into an individual transaction. This approach is inadequate for most complex applications.
To enable transactions, allowing multiple SQL statements to be executed before a COMMIT or ROLLBACK is performed, you must take one of the following two steps:
- Set the MySQL autocommit property or variable to 0. The default setting for AUTOCOMMIT is 1.
- Explicitly initiate a transaction with the START TRANSACTION statement.
Since it is dangerous to assume that the MySQL environment is running with the necessary transaction setting, you should generally include either a SET AUTOCOMMIT=0 or START TRANSACTION statement in any transactional stored program.
The SET autocommit=0 statement simply ensures that MySQL will not implicitly issue a COMMIT after every SQL statement. Note, however, that if you have already initiated a transaction, issuing SET autocommit will have no effect. START TRANSACTION, on the other hand, implicitly commits any currently outstanding changes in your session, terminating the existing transaction and starting a new one.
We recommend that you leave nothing to chance when programming transactions in MySQL stored programs. Therefore, we suggest that you always explicitly commence a transaction with a START TRANSACTION statement and explicitly end your transaction with a COMMIT or ROLLBACK.
|
Example 8-1 shows a transaction implemented in a stored procedure using a SET AUTOCOMMIT statement.
Example 8-1. Commencing a transaction using SET AUTOCOMMIT
CREATE PROCEDURE tfer_funds (from_account int, to_account int,tfer_amount numeric(10,2)) BEGIN SET autocommit=0; UPDATE account_balance SET balance=balance-tfer_amount WHERE account_id=from_account; UPDATE account_balance SET balance=balance+tfer_amount WHERE account_id=to_account; COMMIT; END; |
Example 8-2 shows an example of defining a transaction using START TRANSACTION.
Example 8-2. Commencing a transaction using START TRANSACTION
CREATE PROCEDURE tfer_funds (from_account int, to_account int,tfer_amount numeric(10,2)) BEGIN START TRANSACTION; UPDATE account_balance SET balance=balance-tfer_amount WHERE account_id=from_account; UPDATE account_balance SET balance=balance+tfer_amount WHERE account_id=to_account; COMMIT; END; |
As we've said, transactions normally complete when either a COMMIT or a ROLLBACK statement is executed. However, be aware that some statementsusually Data Definition Language (DDL) statementscan cause implicit COMMITs. The statements that implicitly commit, and should therefore be avoided when a transaction is active, include the following:
ALTER FUNCTION |
ALTER PROCEDURE |
ALTER TABLE |
BEGIN |
CREATE DATABASE |
CREATE FUNCTION |
CREATE INDEX |
CREATE PROCEDURE |
CREATE TABLE |
DROP DATABASE |
DROP FUNCTION |
DROP INDEX |
DROP PROCEDURE |
DROP TABLE |
UNLOCK TABLES |
LOAD MASTER DATA |
LOCK TABLES |
RENAME TABLE |
trUNCATE TABLE |
SET AUTOCOMMIT=1 |
START TRANSACTION |