What is a database transaction and why is it important?

A database transaction represents a coherent set of SQL operations treated as a single unit of work. It adheres to the principles of atomicity, consistency, isolation, and durability (ACID). To illustrate, consider a financial transaction, such as transferring funds between bank accounts, which involves multiple SQL queries:

1. Check the existence of sufficient funds in the source account (SELECT query).

2. Deduct the balance from the source account (UPDATE query).

3. Update the destination account to reflect the deposited amount (UPDATE query).

In this scenario, if a failure occurs after the first two queries are executed but before the third one, it could result in an inconsistent state with money deducted but not deposited. To mitigate such issues, transactions come into play.

Transactions commence with a BEGIN statement, and subsequent SQL queries are executed within the transaction context. Importantly, changes are not permanently applied to the database until a COMMIT statement is issued. If any error or failure arises before the COMMIT, the transaction can be rolled back, ensuring that all queries within the transaction are undone, and data consistency is maintained.

In essence, database transactions provide a robust mechanism to safeguard data integrity and prevent partial or erroneous updates by treating a sequence of SQL queries as an all-or-nothing operation.