SUM « Aggregate Functions « MySQL Tutorial

16.10.1.Using SUM([DISTINCT] expr) function to sum the salary
16.10.2.A SUM() function can be added to CASE statement to give us the total
16.10.3.SUM with NULL value
16.10.4.SUM() function totals the values in a given column.
16.10.5.SUM function with column calculation
16.10.6.Get the average of a set of sums for a grouped table
16.10.7.The SUM() and AVG() aggregate functions do not work with temporal values.