SQL AST: fix aggregate functions with GROUP BY #599
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Aggregate functions that return null on an empty set will instead return no result if the expression has a GROUP BY clause. This currently affects the Avg, Sum, and MinMax return type exts. Also, SUM can always return NULL if there is no GROUP BY: > If there are no matching rows, or if expr is NULL, SUM() returns NULL. https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_sum
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Aggregate functions that return null on an empty set will instead
return no result if the expression has a GROUP BY clause.
This currently affects the Avg, Sum, and MinMax return type extensions.
We also update Sum to always allow a null result without GROUP BY, as per https://dev.mysql.com/doc/refman/8.0/en/aggregate-functions.html#function_sum: