ACID Properties in SQL – Ensuring Reliable Transactions

What Are ACID Properties?

ACID is an acronym that defines the key properties of a transaction in a relational database. These properties ensure that database operations are processed reliably, even in situations involving power failure, system crashes, or multiple simultaneous users.

ACID stands for:

• A – Atomicity
• C – Consistency
• I – Isolation
• D – Durability

Understanding ACID properties is essential for working with SQL transactions, data integrity, and concurrent processing.

1. Atomicity

Definition:

Atomicity ensures that a transaction is all or nothing. Either every step in the transaction completes successfully, or none of them do.

Example:

START TRANSACTION;

UPDATE accounts SET balance = balance - 500 WHERE id = 1;
UPDATE accounts SET balance = balance + 500 WHERE id = 2;

COMMIT;

If one UPDATE fails, ROLLBACK will undo all changes. You never end up with partial updates.

2. Consistency

Definition:

A transaction brings the database from one valid state to another valid state. It must follow all data integrity rules, such as foreign keys, constraints, and data types.

Example:

A transfer from one account to another should not result in the total money in the system increasing or decreasing unexpectedly.

If the system has constraints (e.g., balance cannot be negative), consistency ensures they're always enforced.

3. Isolation

Definition:

Each transaction runs independently of other transactions. Even if multiple transactions occur at the same time, their intermediate steps are not visible to others.

This prevents problems like:

• Dirty reads (reading uncommitted changes)
• Non-repeatable reads
• Phantom reads

Example:

If two users try to book the last ticket simultaneously, isolation ensures only one booking is committed first, avoiding double booking.

4. Durability

Definition:

Once a transaction is committed, its changes are permanent, even if there's a system crash or power failure.

Example:

If a bank transaction is committed, the updated balances must persist even after a server restart.

Why Are ACID Properties Important?

• Prevent data corruption
• Enable safe concurrent access
• Ensure system reliability
• Support recovery after failure

ACID compliance is a critical part of enterprise databases like MySQL (InnoDB), PostgreSQL, Oracle, and SQL Server.