How to Ensure Atomic and Reliable Row Updates in T-SQL?

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:

• It utilizes a more explicit approach to check if a row exists using the IF EXISTS clause, effectively preventing incorrect inserts.
• The update statement ensures that overbooking is prevented by checking if the updated ticket count exceeds the maximum allowed.
• The transaction handling guarantees atomic behavior, ensuring that either both operations occur or neither, maintaining data consistency.
• The transaction result is explicitly returned as 1 (true) if the transaction commits successfully or 0 (false) if it fails, providing the desired return value.

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