Proving SQL query equivalency

Question

How would you go about proving that two queries are functionally equivalent, eg they will always both return the same result set.

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As I had a specific query in

Answer 1

1) Real equivalency proof with Cosette:
Cosette checks (with a proof) if 2 SQL query's are equivalent and counter examples when not equivalent. It's the only way to be absolutely sure, well almost ;) You can even throw in 2 query's on their website and check (formal) equivalence right away.

Link to Cosette: http://cosette.cs.washington.edu/

Link to article that gives a good explanation of how Cosette works: https://medium.com/@uwdb/introducing-cosette-527898504bd6


2) Or if you're just looking for a quick practical fix:
Try this stackoverflow answer: [sql - check if two select's are equal]
Which comes down to:

(select * from query1 MINUS select * from query2) 
UNION ALL
(select * from query2 MINUS select * from query1)

This query gives you all rows that are returned by only one of the queries.