Computing Totals and Subtotals
You can use other aggregate functions to obtain summarizations and totals of data in your tables.
All the modifiers you’ve learned to use for the COUNT function are available for any other aggregate function.
You have to obtain totals to answer questions like: “How much money did we get from sales?” or “How many units have we sold of this product?” You could count records using a custom application. However, you will be able to get the desired information more efficiently by directly querying the database.
The SUM function does exactly what you would expect: it yields the sum of values ...