Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

Aggregate Functions

Aggregates are functions you can use to get summary values. You apply aggregates to sets of rows: to all the rows in a table, to just those rows specified by a WHERE clause, or to groups of rows set up in the GROUP BY clause (discussed in the next chapter). No matter how you structure the sets, you get a single value for each set of rows.

Take a look at the difference in the results of the following two queries: The first finds each individual yearly sale value in the titles table (one sale listing per row); the second calculates the total yearly sales for all books in the titles table (one total sale listing per set, and the table is the set).

SQL

select ytd_sales from titles ytd_sales =========== 4095 4095 3336 4072 3876 ...