Explain Transaction Isolation Levels.

Transaction isolation levels define how one database transaction is isolated from others, controlling visibility of changes and ensuring consistency under concurrent workloads.

When to Use

Use them when multiple transactions run at the same time. For example, banking requires strict isolation for accuracy, while analytics queries may prefer weaker isolation for speed.

Example

If one transaction transfers money and another checks the balance, a low isolation level may show a dirty read, while a higher one ensures the final committed balance only.

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Why Is It Important

Isolation levels prevent anomalies like dirty reads, non-repeatable reads, and phantom reads. They directly affect how trustworthy and consistent your database remains under concurrency.

Interview Tips

Be ready to name the four SQL standards — Read Uncommitted, Read Committed, Repeatable Read, Serializable — and link each to the anomalies they prevent. Many databases default to Read Committed.

Trade-offs

Higher levels (Serializable) maximize consistency but reduce concurrency. Lower levels boost performance but risk inconsistent data. The right choice depends on your workload.

Pitfalls

Avoid assuming stricter is always better — high isolation can cripple throughput.

Also, don’t use weaker levels blindly; dirty reads may silently introduce business logic errors.