sql tips: you can group by anythingpic.twitter.com/YoJVlUyghG
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That is, given SELECT a AS b FROM sometable GROUP BY b; if "sometable" has a column "b", the GROUP BY clause refers to that column, and NOT to "a". If it has no such column, the query is not valid per the spec.
Accordingly, if a database allows GROUP BY alias as an extension, you have the possibility that adding a column to the base tables might silently change the meaning of the query, since GROUP BY real_column must be preferred over GROUP BY alias if both exist.
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