The SUM function returns the summation total of a collection of numbers.
The expression is a column name, numeric expression, or literal that only involves numeric values or nulls. Null values are removed.
The sum of no rows is NULL, not zero.
DISTINCT will remove duplicate values.
The result is only as precise as the data types used in the expression.

                                                  SUM(expression)

                                                             or

                                        SUM(DISTINCT column-name)