SQL transactions are a type of database operation in which multiple SQL statements are executed as a single unit. Transactions are often used to ensure data integrity, by ensuring that all of the SQL statements in a transaction are either committed or rolled back.
If a transaction is committed, then all of the SQL statements in the transaction are executed. If a transaction is rolled back, then none of the SQL statements in the transaction are executed.
Transactions can be nested, meaning that a transaction can contain another transaction. When a nested transaction is committed, all of the SQL statements in both the nested transaction and the outer transaction are committed. If a nested transaction is rolled back, then all of the SQL statements in the nested transaction are rolled back, but the SQL statements in the outer transaction are not affected.
SQL transactions are a way to ensure data integrity in a database. They are used to group together multiple SQL statements so that they can be treated as a single unit. If any of the statements in a transaction fail, the entire transaction will be rolled back and the database will be restored to its previous state.
Transactions are useful for ensuring that data is consistent across multiple tables or even across multiple databases. For example, if you are transferring money from one bank account to another, you would want to use a transaction so that either both account balances are updated or neither is updated.
To start a transaction, you use the BEGIN TRANSACTION statement. To commit the transaction and make all the changes permanent, you use the COMMIT TRANSACTION statement. To roll back the transaction and undo all the changes, you use the ROLLBACK TRANSACTION statement.
SQL provides different levels of transaction isolation, which determine how data within a transaction is visible to other transactions. The most common isolation levels are read committed, repeatable read, and serializable.
Transaction isolation levels can be controlled at the database or table level. For example, a database can be configured to use the read committed isolation level for all transactions. Alternatively, a table can be configured to use the serializable isolation level, while all other tables use the read committed isolation level.
A rollback transaction is a transaction that is undone. This can be done for a number of reasons, but most often it is done because there was an error in the transaction that needs to be corrected. A rollback can also be done if the transaction is no longer needed or if it needs to be changed for some reason.