How Can I Implement Automated Rollback on Exceptions in MySQL Transactions?

MySQL Transaction Handling: Rolling Back on Exceptions

In the context of MySQL, transactions encompass a series of database operations that are executed as a single logical unit. By default, if any of the commands within a transaction throws an error, the entire transaction is rolled back, ensuring data integrity. However, there are situations where one might want to explicitly control the rollback behavior.

One method to achieve automated rollback on any exception is through the use of MySQL's DECLARE ... HANDLER syntax. This allows a user to specify a handler function that is executed if a SQLEXCEPTION is encountered during transaction execution.

To demonstrate how this works, consider the following example:

DELIMITER $$

CREATE PROCEDURE `sp_fail`()
BEGIN
    DECLARE `_rollback` BOOL DEFAULT 0;
    DECLARE CONTINUE HANDLER FOR SQLEXCEPTION SET `_rollback` = 1;
    START TRANSACTION;
    INSERT INTO `tablea` (`date`) VALUES (NOW());
    INSERT INTO `tableb` (`date`) VALUES (NOW());
    INSERT INTO `tablec` (`date`) VALUES (NOW()); -- FAIL
    IF `_rollback` THEN
        ROLLBACK;
    ELSE
        COMMIT;
    END IF;
END$$

DELIMITER ;

In this procedure, we declare a variable _rollback to flag if any error occurred. A handler is defined to set this flag to 1 when a SQLEXCEPTION is encountered. We then execute the desired database operations within the transaction. After all operations are attempted, we check the _rollback flag and issue a ROLLBACK only if an error occurred. Otherwise, we COMMIT the transaction.

By using this technique, we can ensure that any transaction-level error triggers a rollback, preserving the desired behavior of keeping the database in a consistent state.

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