MySQL TransactionsReggie Niccolo Santos

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Outline

What are transactions? Sample scenario Properties (ACID) Syntax Example

What are transactions?

Sequential group of DML statements, which is performed as if it were one single work unit

Will never complete unless each individual operation within the group is successful. If any operation within the transaction fails, the entire transaction will fail

What are transactions?

Begins with the first executable SQL statement

Ends when it is committed or rolled back, either explicitly with a COMMIT or ROLLBACK statement or implicitly when a DDL statement is issued

Sample Scenario

Suppose a bank customer transfers money from his savings account (SB a/c) to his current account (CA a/c), the statement will be divided into four blocks:

Debit SB a/c Credit CA a/c Record in Transaction Journal End Transaction

Sample Scenario

The SQL statement to debit SB a/c is as follows:

UPDATE sb_accountsSET balance = balance - 1000WHERE account_no = 932656;

Sample Scenario

The SQL statement to credit CA a/c is as follows:

UPDATE ca_accountsSET balance = balance + 1000WHERE account_no = 933456;

Sample Scenario

The SQL statement for recording in the transaction journal is as follows:

INSERT INTO journal VALUES (100896, 'Transaction on Benjamin Hampshair a/c', '26-AUG-08' 932656, 933456, 1000);

Sample Scenario

The SQL statement for End Transaction is as follows:

COMMIT WORK;

Properties (ACID)

Atomicity Ensures that all operations

within the work unit are completed successfully; otherwise, the transaction is aborted at the point of failure and previous operations are rolled back to their former state

Properties (ACID)

Consistency Ensures that the database

properly changes states upon a successfully committed transaction

Properties (ACID)

Isolation Enables transactions to operate

independently of and transparent to each other

Properties (ACID)

Durability Ensures that the result or effect

of a committed transaction persists in case of a system failure

Syntax

START TRANSACTION [transaction_characteristic [, transaction_characteristic] ...]

transaction_characteristic: WITH CONSISTENT SNAPSHOT | READ WRITE | READ ONLY

BEGIN [WORK]COMMIT [WORK] [AND [NO] CHAIN] [[NO] RELEASE]ROLLBACK [WORK] [AND [NO] CHAIN] [[NO] RELEASE]

SET autocommit = {0 | 1}

Example

START TRANSACTION;

SELECT @A:=SUM(salary) FROM employee_salary WHERE type=1;

UPDATE salary_report SET summary=@A WHERE type=1;

COMMIT;

References Http://www.tutorialspoint.com/mysql/mysql-transa

ctions.htm Http://www.w3resource.com/mysql/mysql-transacti

on.php