Friday, September 14, 2007

sum(field_with_all_null) == null

http://www.oracle.com/technology/oramag/oracle/05-jul/o45sql.html shows how aggregate functions react to empty sets ie "no entry except nulls".

COUNT returns zero whereas AVG, SUM, MAX, and MIN return null.

If you have no values to count, it's fair to say that you have zero values, whereas you can't really come up with, say, a maximum value without at least one value from which to choose. We can make a reasonable argument that SUM should return zero instead of null, that the sum of no values is zero, but Oracle's implementation of the behavior we describe here is fully compliant with the SQL standard.

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