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An UPSERT Pattern to Avoid

Aaron Bertrand doesn’t like a common insert/update pattern:

I think everyone already knows my opinions about MERGE and why I stay away from it. But here’s another (anti-)pattern I see all over the place when people want to perform an upsert (update a row if it exists and insert it if it doesn’t):

IF EXISTS (SELECT 1 FROM dbo.t WHERE [key] = @key) BEGIN
UPDATE dbo.t SET val = @val WHERE [key] = @key;
INSERT dbo.t([key], val) VALUES(@key, @val);

This looks like a pretty logical flow that reflects how we think about this in real life:

Does a row already exist for this key?
YES: OK, update that row.
NO: OK, then add it.

Click through to learn why this is a bad idea.