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Returns the total of the specified expression for each group of rows.
SUM ( expression | DISTINCT column-name )
expression The object to be summed. This is commonly a column name.
DISTINCT column-name Computes the sum of the unique values in column-name for each group of rows. This is of limited usefulness, but is included for completeness.
The following statement returns the value 3749146.740.
SELECT SUM( salary ) FROM employee
Rows where the specified expression is NULL are not included.
Returns NULL for a group containing no rows.
