Name the four levels of transaction isolation.

Four standard transaction isolation levels in SQL:

1. READ UNCOMMITTED

   • Possible to read uncommitted data ("dirty" read).
   • Possible to read data that no longer exists ("phantom" read).
   • Possible to read the same data multiple times with different values ("non-repeatable" read).

2. READ COMMITTED

   • Prevents "dirty" read.
   • Allows "non-repeatable" read.
   • Allows "phantom" read.

3. REPEATABLE READ

   • Prevents "dirty" and "non-repeatable" reads.
   • Allows "phantom" read.

4. SERIALIZABLE

   • The strictest level.
   • Prevents "dirty", "non-repeatable" and "phantom" reads.
   • Transactions are executed as if they are serialized (one after another).