SQL defines several transaction isolation levels that control the level of locking and consistency in a transaction. The four isolation levels defined by the SQL standard are:
Read uncommitted: This is the lowest isolation level, where a transaction can read uncommitted changes made by other transactions. This level allows for dirty reads, non-repeatable reads, and phantom reads.
Read committed: This isolation level ensures that a transaction only reads committed data, avoiding dirty reads. However, non-repeatable reads and phantom reads can still occur.
Repeatable read: This level guarantees that a transaction will always see the same committed data, even if other transactions update the data in between reads. However, phantom reads are still possible.
Serializable: This is the highest isolation level, where transactions are executed as if they are the only transactions running in the system. This level prevents dirty reads, non-repeatable reads, and phantom reads, but can be more restrictive on concurrency.
Different database management systems may provide additional isolation levels beyond the SQL standard, such as snapshot isolation or read stability. The choice of isolation level depends on the specific requirements of the application and the tradeoff between concurrency and consistency.
