Isolation levels in MySQL

Introduction

Isolation levels control how much one transaction is separated from others. This helps keep data correct when many people use the database at the same time.

When you want to avoid errors caused by multiple people changing data at once.

When you need to balance speed and accuracy in your database.

When you want to prevent reading data that is not yet final.

When you want to control how transactions see changes made by others.

When you want to avoid problems like dirty reads or lost updates.

Syntax

MySQL

SET TRANSACTION ISOLATION LEVEL level_name;

Replace level_name with one of the standard levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE.

This command sets the isolation level for the current session or next transaction.

Examples

This level allows reading uncommitted changes from other transactions (fast but risky).

MySQL

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

This level only reads data that has been committed by others (common default).

MySQL

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

This level ensures that if you read data twice in the same transaction, it stays the same.

MySQL

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

This is the strictest level, making transactions appear one after another to avoid conflicts.

MySQL

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Sample Program

This example starts a transaction, sets the isolation level to REPEATABLE READ, checks the current isolation level, then commits.

MySQL

START TRANSACTION;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT @@transaction_isolation;
COMMIT;

Important Notes

Higher isolation levels reduce errors but can slow down the database.

MySQL default isolation level is REPEATABLE READ.

Changing isolation levels affects only the current session or transaction unless set globally.

Summary

Isolation levels control how transactions see and affect data.

Four main levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE.

Choose the level based on your need for speed versus data accuracy.