In this tutorial, we will learn about MySQL Transaction statements. Transaction statements allow the ability to run multiple statements as a single unit, in such way that either all or none of the database statements succeed.
A transaction executes one or more SQL statements as a single unit of work, in such a way that either all the statements succeed or none of them will execute in case of an error.
If all the statements succeed the transaction is committed and the changes are recorded in the database permanently. If an error occurs during the transaction, all the changes by the transaction are undone and ignored. The database is left in the state it was prior to the start of the failed transaction.
Transaction statements and the description are as follows:
|START TRANSACTION||This statement begins a new transaction.|
|COMMIT||This statement commits the current transaction and makes the changes permanent to the database. We can use COMMIT to store the changes to the disk.|
|ROLLBACK||This statement rollbacks the current transaction and cancels the changes. We can use ROLLBACK to ignore the changes made by the transaction.|
|SET AUTOCOMMIT||This statement is used to disable/enable the default autocommit mode for the current database connection.|
By default, MySQL server runs with autocommit mode enabled. When this mode is set, as soon as we execute SQL statement that updates a tables, MYSQL stores the updates on the disk. i.e each statement is considered as a transaction.
To disable autocommit mode for series of SQL statements, we can use the START TRANSACTION statement.
— SQL statement1
— SQL statement2
The autocommit mode remains disabled until you end the transaction with either COMMIT or ROLLBACK. We can check the current setting by using the SELECT statement as follows:
mysql> SELECT @@AUTOCOMMIT;
| @@AUTOCOMMIT |
| 1 |
1 row in set (0.00 sec)
Let’s look at a simple transaction example. A transfer of $500 money from savings account of customer A to account of customer B.
A successful transaction should do the following things :
- Check for sufficient balance in customer’s account of A.
- Withdraw $500 from the account A.
- Deposit $500 into account B.
Pseudocode for the transaction:
Check for sufficient balance in account A
If balance > 500
Withdraw money from the Account A.
Deposit money into Account B.
COMMIT — if no errors make changes permanent
ROLLBACK — if something went wrong or insufficient balance.
— end transaction
With transactional-safe storage engine like InnoDB we can disable autocommit with the following command:
In this mode, we must use the COMMIT to make the changes permanent after successful transaction or ROLLBACK to ignore the changes made since the start of the failed transaction.
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