Locking Mechanisms in SQL – Ensuring Safe Data Access

What Is Locking in SQL?

Locking in SQL is a mechanism used by the database to manage concurrent access to data. It ensures that multiple users can read and write data safely and consistently, without conflicts or data corruption.

Locking is a key component of maintaining ACID properties—especially isolation—in SQL transactions.

Why Is Locking Needed?

• To prevent dirty reads, lost updates, and data inconsistency
• To control concurrent access to the same data
• To ensure transactional integrity

Types of Locks in SQL

1. Shared Lock (Read Lock)

• Allows multiple users to read a resource (row/table) simultaneously
• No one can write to the data until the lock is released
• Example Use: SELECT statements in some isolation levels

2. Exclusive Lock (Write Lock)

• Only one user can write or modify the data
• Other users cannot read or write until the lock is released
• Example Use: UPDATE, DELETE, INSERT

Lock Granularity

SQL databases use different levels of locking:

Example with Transaction and Locking

START TRANSACTION;

SELECT * FROM products WHERE product_id = 1 FOR UPDATE;

UPDATE products SET stock = stock - 1 WHERE product_id = 1;

COMMIT;

FOR UPDATE acquires an exclusive lock on the row so no other transaction can modify it until the current transaction is committed or rolled back.

Common Locking Issues

Tips for Managing Locks

• Keep transactions short and fast
• Always access tables in the same order
• Use indexes to reduce locking overhead
• Avoid user input inside transactions (it increases lock time)
• Use appropriate isolation levels

Locking in Different SQL Engines