How Can I Atomically Check for Row Existence and Insert in T-SQL?

How to Perform Atomic Row Existence Checks and Inserts Using T-SQL

In database systems, it is often necessary to perform operations on rows that may or may not exist. In such scenarios, it is crucial to ensure atomicity and reliability to maintain data integrity.

To check if a row exists and perform an insert operation accordingly, consider the following approach in T-SQL:

IF EXISTS (SELECT * FROM Bookings WHERE FlightID = @Id)
BEGIN
    -- Update the existing row here...
END
ELSE
BEGIN
    -- Insert a new row here...
END

This approach ensures atomicity by using an IF EXISTS statement to perform both existence checks and insert operations within a single transaction. It prevents race conditions where concurrent transactions could potentially insert duplicate rows.

Transaction Control

To wrap the operation within a transaction, use statements such as BEGIN TRANSACTION, COMMIT TRANSACTION, and ROLLBACK TRANSACTION.

Row Count Handling

The @@ROWCOUNT global variable tracks the number of rows affected by an operation. In this case, it is not reliable for existence checks as an update operation may return 0 rows even when a row exists (e.g., if no changes are applied). Therefore, it is more reliable to use IF EXISTS with a subquery.

Returning Transaction Status

To indicate the success of the transaction, you can use a TRY-CATCH block and return an appropriate value:

BEGIN TRY
    -- Transaction Logic
    COMMIT TRANSACTION
    RETURN 1 -- Transaction committed
END TRY
BEGIN CATCH
    ROLLBACK TRANSACTION
    RETURN 0 -- Transaction failed
END CATCH

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