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Transactions and database locking

Transactions and database locking are important concepts in relational databases that are used to ensure the consistency and reliability of data.

  • Transactions: A transaction is a sequence of one or more database operations that are executed as a single unit of work. Transactions ensure that either all of the operations are completed, or none of them are completed, in order to maintain the consistency of the data.

Here's an example of a transaction in SQL:

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BEGIN TRANSACTION;
UPDATE employees
SET salary = salary + 5000
WHERE department = 'Sales';
COMMIT;

In this example, the BEGIN TRANSACTION statement starts a new transaction. The UPDATE statement that follows updates the salaries of all employees in the Sales department. The COMMIT statement at the end of the transaction confirms that all of the changes made during the transaction should be permanently committed to the database.

  • Database Locking: Database locking is a mechanism that is used to ensure that multiple transactions can access the database without causing inconsistencies or data corruption. When a transaction accesses a piece of data, it locks the data to prevent other transactions from modifying it until the first transaction is completed.

For example, consider the following scenario:

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Transaction 1: BEGIN TRANSACTION;
               SELECT * FROM employees WHERE employee_id = 1;
               -- Transaction 1 now has a lock on the row with employee_id = 1.

Transaction 2: BEGIN TRANSACTION;
               -- Transaction 2 cannot modify the row with employee_id = 1 because it is locked by Transaction 1.

In this scenario, Transaction 1 has locked the row with employee_id equal to 1. Transaction 2 cannot modify the locked data until Transaction 1 is completed. This helps to prevent inconsistencies and corruption in the data.

In general, database locking is an important mechanism for ensuring the consistency and reliability of data in relational databases. However, excessive locking can also cause performance issues, so it's important to carefully consider the trade-offs when designing and implementing locking strategies.



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