SQL transactions are a crucial aspect of database management systems (DBMS) that ensure the integrity, consistency, and reliability of data. A transaction in SQL represents a sequence of one or more SQL statements that are executed as a single unit of work. The fundamental properties of a transaction, often referred to as ACID properties, are Atomicity, Consistency, Isolation, and Durability.
Atomicity ensures that a transaction is treated as a single, indivisible unit of work. Either all the changes made by the transaction are committed to the database, or none of them are. If any part of the transaction fails, the entire transaction is rolled back to its previous state.
Consistency ensures that a transaction brings the database from one valid state to another. The database must satisfy a set of integrity constraints before and after the transaction. If the transaction violates any of these constraints, it is rolled back to maintain a consistent state.
Isolation ensures that the execution of one transaction is isolated from the execution of other transactions. Even though multiple transactions may be executing concurrently, the final result should be as if the transactions were executed serially. Isolation prevents interference between transactions and ensures data integrity.
Durability guarantees that once a transaction is committed, its effects are permanent and survive any subsequent failures, such as a system crash. The changes made by a committed transaction are stored in non-volatile memory, typically on disk, so that they can be recovered in case of a system failure.
In SQL, transactions are typically managed using the following statements:
BEGIN TRANSACTION: Marks the beginning of a transaction.
COMMIT: Commits the transaction, making all changes made during the transaction permanent.
ROLLBACK: Undoes the changes made during the current transaction and restores the database to its state before the transaction began.
SAVEPOINT: Sets a point within a transaction to which you can later roll back.
SET TRANSACTION: Specifies characteristics for the transaction, such as isolation level and access mode.
BEGIN TRANSACTION; UPDATE Accounts SET Balance = Balance - 100 WHERE AccountID = 1; UPDATE Accounts SET Balance = Balance + 100 WHERE AccountID = 2; COMMIT;
In this example, the two updates are part of a single transaction. If either update fails (due to, for example, a constraint violation or system error), the entire transaction will be rolled back, ensuring the atomicity of the operation. Transactions play a critical role in maintaining the integrity and reliability of data in SQL databases, particularly in scenarios with concurrent access and multiple users.