If EXISTS, THEN SELECT ELSE INSERT AND THEN SELECT

Handling concurrency & transaction control

For robust handling of transactions and to prevent potential deadlocks or race conditions, mind the transaction level and apply explicit locking mechanisms. Initially, check the existence of a record using SELECT with NOLOCK hint to lessen locking overhead:

BEGIN TRANSACTION;
IF NOT EXISTS (SELECT 1 FROM MyTable WITH (NOLOCK) WHERE MyColumn = 'MyValue') -- Like ninja, it skips locks
    INSERT INTO MyTable (MyColumn) VALUES ('MyValue');
SELECT * FROM MyTable WHERE MyColumn = 'MyValue';
COMMIT TRANSACTION;
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Retrieving new identity after insert

After a record insertion, retrieving the identifier swiftly and efficiently is key. The SCOPE_IDENTITY() function returns the last identity value that is generated:

SELECT SCOPE_IDENTITY() AS 'Identity'; -- Like asking, 'Hey SQL, who was the last one in?'
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Remember, the TableID column should be an identity column for SCOPE_IDENTITY() to function correctly.

Duplication handling

Applying a UNIQUE constraint on relevant columns prevents the insertion of duplicate data. If such a constraint exists, gracefully handle potential insertion failures:

BEGIN TRY
    BEGIN TRANSACTION;
    ...
    COMMIT TRANSACTION;
END TRY
BEGIN CATCH
    IF ERROR_NUMBER() = 2627 -- Unique constraint error code
        SELECT * FROM MyTable WHERE MyColumn = 'MyValue';
    ELSE
        ROLLBACK TRANSACTION; -- Like hitting the undo button for the transaction
        RAISERROR('An unexpected error occurred', 16, 1); -- Like yelling, 'Mayday! Mayday!'
    END CATCH;
END CATCH
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Solutions across different SQL flavors

Different database systems offer unique ways to handle 'check, then insert if not exists,' aka upsert operations. Let's compare:

1. SQL Server MERGE: Handles multiple DML operations in a single statement.
2. PostgreSQL UPSERT: Leverages ON CONFLICT clause on INSERT statement to gracefully handle potential duplicates.
3. MySQL's INSERT ON DUPLICATE: Useful when the new record may duplicate the primary key or unique constraint.

Preserving data integrity & boosting performance

Including a UNIQUE constraint helps maintain data integrity preventing duplicates. For optimal query efficiency and less data transfer overhead, specify only required columns in your SELECT statement:

SELECT TableID, MyColumn FROM MyTable WHERE MyColumn = 'MyValue'; -- It's like opting for a lighter travel backpack instead of a heavy suitcase
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