Joe Celko's SQL for Smarties, 3rd Edition

Chapter 21. Aggregate Functions

ONE OF THE MAJOR purposes of a database system is to turn data into information. This usually means doing some statistical summary from that data. Descriptive statistics measure some property of an existing data set and express it as a single number. Though there are very sophisticated measures, most applications require only basic, well-understood statistics. The most common summary functions are the count (or tally), the average (or arithmetic mean), and the sum (or total). This minimal set of descriptive statistical operators is built into the SQL language, and vendors often extend these options with others. These functions are called set functions in the ANSI/ISO SQL standard, but vendors, textbook writers, ...