Good question, thanks. I have caused this error many times while composing my queries, so when I realized there was an alias in the GROUP BY clause, I figured that had to be the answer.
- webrunner
(Edit) P.S. Thanks, Hugo, for the detailed explanation of why this error happens.
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A SQL query walks into a bar and sees two tables. He walks up to them and asks, "Can I join you?"
Ref.: http://tkyte.blogspot.com/2009/02/sql-joke.html