• In MySQL, only databases or tables that use the Innodb database engine support transactions.

• Transaction processing can be used to maintain the integrity of the database and ensure that batch SQL statements are either executed or not executed.

• Transactions are used to manage insert, update and delete statements

Generally speaking, transactions must meet four conditions (ACID): Atomicity (or indivisibility), Consistency, Isolation (also known as independence) and persistence.

• **Atomicity: * * all operations in a transaction are either completed or not completed, and will not end in an intermediate phase. If an error occurs during the execution of the transaction, it will be rolled back to the state before the transaction starts, just as the transaction has never been executed.

• **Consistency: * * the integrity of the database is not destroyed before and after the transaction. This means that the written data must fully comply with all preset rules, including the accuracy and serialization of the data, and the subsequent database can spontaneously complete the predetermined work.

• **Isolation: * * the database allows multiple concurrent transactions to read, write and modify their data at the same time. Isolation can prevent data inconsistency caused by cross execution when multiple transactions are executed concurrently. Transaction isolation is divided into different levels, including Read uncommitted, read committed, repeatable read, and Serializable.

• **Persistence: * * after the transaction is completed, the data modification is permanent and will not be lost even if the system fails.

Under the default setting of MySQL command line, transactions are automatically committed, that is, the COMMIT operation will be executed immediately after the SQL statement is executed. Therefore, to explicitly start a transaction, you must use the command BEGIN or START TRANSACTION, or execute the command SET AUTOCOMMIT=0 to prohibit the automatic submission of the current session.

Transaction control statement:

• BEGIN or START TRANSACTION explicitly starts a transaction;

• COMMIT WORK can also be used for COMMIT, but they are equivalent. COMMIT will COMMIT the transaction and make all modifications to the database permanent;

• ROLLBACK WORK can also be used for ROLLBACK, but they are equivalent. ROLLBACK will end the user's transaction and undo all ongoing uncommitted modifications;

• SAVEPOINT identifier, SAVEPOINT allows to create a SAVEPOINT in a transaction, and there can be multiple savepoints in a transaction;

• RELEASE SAVEPOINT identifier deletes the savepoint of a transaction. When there is no specified savepoint, an exception will be thrown when the statement is executed;

• ROLLBACK TO identifier rolls back the transaction to the marked point;

• SET TRANSACTION is used to set the isolation level of transactions. The InnoDB storage engine provides transaction isolation levels such as READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ and SERIALIZABLE.

There are two main methods for MYSQL transaction processing:

1. It is implemented with begin, rollback and commit

• BEGIN # start a transaction

• ROLLBACK transaction ROLLBACK

• COMMIT transaction confirmation

2. Directly use SET to change the automatic submission mode of MySQL:

• SET AUTOCOMMIT=0 ， automatic submission is prohibited

• SET AUTOCOMMIT=1 ￠ enable auto submit

Transaction test

mysql> use RUNOOB;

Database changed

mysql> CREATE TABLE runoob_transaction_test( id int(5)) engine=innodb;  # Create data table

Query OK, 0 rows affected (0.04 sec)

 

mysql> select * from runoob_transaction_test;

Empty set (0.01 sec)

 

mysql> begin;  # Start transaction

Query OK, 0 rows affected (0.00 sec)

 

mysql> insert into runoob_transaction_test value(5);

Query OK, 1 rows affected (0.01 sec)

 

mysql> insert into runoob_transaction_test value(6);

Query OK, 1 rows affected (0.00 sec)

 

mysql> commit; # Commit transaction

Query OK, 0 rows affected (0.01 sec)

 

mysql>  select * from runoob_transaction_test;

+------+

| id   |


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