Syntax
SUM ( [ DISTINCT | ALL ] Expression )
The
DISTINCT and ALL qualifiers eliminate or retain duplicates. ALL is assumed
if neither ALL nor DISTINCT is specified. For example, if a column contains
the values 1, 1, 1, 1, and 2, SUM(col) returns a greater value than SUM(DISTINCT
col).
Only one DISTINCT aggregate expression per
SelectExpression is
allowed. For example, the following query is not allowed:
SELECT AVG (DISTINCT flying_time), SUM (DISTINCT miles)
FROM Flights
The Expression can contain multiple
column references or expressions, but it cannot contain another aggregate
or subquery. It must evaluate to a built-in numeric data type. If an expression
evaluates to NULL, the aggregate skips that value.
The resulting data
type is the same as the expression on which it operates (it might overflow).