If you use an aggregate function in the select list of an SQL statement, then either all other columns occurring in the select list must also be referenced by means of aggregate functions or their column name must appear in a GROUP BY clause. For example, the following statement uses an aggregate function and a column in the select list and references the column name in the GROUP BY clause:
SELECT COUNT(*), Product_ID FROM Sales_Table GROUP BY Product_ID;
The reason for this is that aggregates return only one value, while a non-GROUP BY column reference can return any number of values.