Ensuring Atomic and Reliable Row Updates in T-SQL
In the realm of data management, it's often crucial to ascertain the existence of a row before performing updates. This is particularly vital in scenarios where data integrity and atomicity are paramount. This article explores a reliable approach to check if a row exists in a table and, if absent, insert it, all within the scope of a single transaction.
Atomic and Reliable Row Updates
Consider a booking system where it's imperative to prevent overbooking and guarantee the reliability of each booking. Implementing this functionality requires an atomic and reliable approach, ensuring that either both operations (update or insert) are successfully executed or both are rolled back, maintaining data integrity.
Examining the Code
The provided code snippet attempts to update a row in the Bookings table. However, if the row doesn't exist, it falls short of handling the scenario correctly. Additionally, the query's usage of @@ROWCOUNT to check for existing rows introduces data inconsistency issues.
Improved Code Implementation
To rectify these issues and ensure the desired behavior, a modified code is presented below:
-- Initiate transaction (assuming SQL Server) BEGIN TRANSACTION -- Attempt to update row UPDATE Bookings SET TicketsBooked = TicketsBooked + @TicketsToBook WHERE FlightId = @Id AND TicketsMax >= (TicketsBooked + @TicketsToBook) -- Check if row updated IF @@ROWCOUNT = 0 BEGIN -- Insert new row (if row doesn't exist) INSERT INTO Bookings ... (omitted) END -- Transaction completion (commit or rollback based on result) IF @@ERROR = 0 BEGIN COMMIT TRANSACTION SELECT 1 AS Result -- Return true (if transaction committed) END ELSE BEGIN ROLLBACK TRANSACTION SELECT 0 AS Result -- Return false (if transaction failed) END
Explanation
This enhanced code includes several crucial modifications:
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