Can you explain what the transaction isolation level means in a database and why it is needed?

The transaction isolation level determines how much a transaction can see the changes made by other concurrent transactions. This is important for preventing issues such as:

• Dirty read — reading uncommitted data.
• Non-repeatable read — data changes between two reads within the same transaction.
• Phantom read — appearance of new rows when re-executing a query.

Main isolation levels:

• Read Uncommitted — the lowest, allows dirty reads.
• Read Committed — prevents dirty reads.
• Repeatable Read — guarantees that data does not change during repeated reads.
• Serializable — the strictest, provides full isolation as if transactions were executed sequentially.

Choosing an isolation level is a balance between performance and data integrity.