How to group on calculation or calculated expression?

Until Firebird 2, you could only group on regular columns and UDF results. With Firebird 2 you can group by on any column in the result set, simply by using it's field number, example:

SELECT c1*10, min(c2)
FROM t1
GROUP BY c1*10;

doesn't work. But, you can use:

SELECT c1*10, min(c2)
FROM t1
GROUP BY 1;

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