SQL Transactions and Isolation Levels: A Practical Guide
Transactions protect data integrity, but isolation levels change what they actually guarantee. Here is the hard way. Transactions are how databases keep data consistent when multiple operations must succeed or fail together. I learned their importance the hard way, after a partial update left an order record without its line items because the second query failed. After that incident, I treated transactions as a default, not an optimization. Understanding isolation levels came later, when concurrent transactions started producing results that did not match my mental model. The ACID Properties Transactions guarantee four properties, known by the acronym ACID. Atomicity means all changes happen or none do. Consistency means the database moves from one valid state to another. Isolation means concurrent transactions do not interfere with each other. Durability means committed changes survive crashes. The first and last are easy to appreciate. Isolation is where most of the complexity lives. Starting and Committing A transaction begins with BEGIN or START TRANSACTION and ends with COMMIT or ROLLBACK . Every statement between them is part of the same transaction. If any statement fails, I roll back to undo all…