A database transaction is a fundamental concept in database management systems (DBMS) that ensures the integrity and consistency of data within a database. It represents a single logical unit of work or a sequence of one or more SQL statements that are executed as a whole. The key aspects of database transactions are:
1. *ACID Properties:*
Transactions adhere to the ACID properties, which stand for Atomicity, Consistency, Isolation, and Durability. These properties ensure that the database remains in a reliable state even in the presence of errors, hardware failures, or concurrent access by multiple users.
*Atomicity:* A transaction is atomic, meaning it is treated as a single, indivisible unit. Either all its operations are executed, or none of them are. If any part of the transaction fails, the entire transaction is rolled back to its original state.
*Consistency:* A transaction brings the database from one consistent state to another. The database must satisfy integrity constraints and business rules before and after the transaction.
*Isolation:* Transactions are executed in isolation from one another to prevent interference. This ensures that the intermediate states of one transaction are not visible to other transactions until the first transaction is committed.
*Durability:* Once a transaction is committed, its effects on the database are permanent and will survive system crashes or failures. The changes are stored persistently.
2. *Transaction Lifecycle:*
A transaction typically follows the following lifecycle:
*BEGIN:* A transaction begins with the `BEGIN TRANSACTION` or `START TRANSACTION` statement. After this point, any SQL statements executed are considered part of the transaction.
*EXECUTION:* The SQL statements within the transaction are executed one after the other.
*COMMIT:* If all statements within the transaction execute successfully and meet the consistency requirements, the transaction is committed using the `COMMIT` statement. At this point, the changes made by the transaction become permanent.
*ROLLBACK:* If an error occurs during the transaction or if the transaction fails to meet consistency requirements, it is rolled back using the `ROLLBACK` statement. This reverts the database to its state before the transaction started.
3. *Concurrency Control:*
Database management systems must handle multiple transactions executing concurrently. Concurrency control mechanisms like locking, timestamp-based ordering, and isolation levels (e.g., READ COMMITTED, SERIALIZABLE) ensure that concurrent transactions do not interfere with each other and maintain data integrity.
4. *Savepoints:*
Some DBMSs support the concept of savepoints within a transaction. Savepoints allow you to mark a point in a transaction's execution, and you can later roll back to that point while preserving the changes made after it.
In summary, database transactions are essential for ensuring data integrity and consistency in a DBMS. They follow the ACID properties, provide a structured way to group SQL statements into logical units of work, and handle concurrency control to enable multiple transactions to run simultaneously while maintaining data integrity.
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