I often refactor SQL, cleaning up convoluted statements using language features such as views, common table expressions and nested joins. It’s one thing to rework a gnarly query into something legible; it’s another thing to verify that the rewritten query returns the same data as the original.

The starting point for verification is comparing the result sets returned by the old and new queries. I’ve recently found Microsoft T-SQL’s EXCEPT operator quite handy for this purpose. Continue reading →