SUM( ) and AVG( ) produce the total and average (mean) of a set of values:
What is the total amount of mail traffic and the average size of each message?
mysql> SELECT SUM(size) AS 'total traffic', -> AVG(size) AS 'average message size' -> FROM mail; +---------------+----------------------+ | total traffic | average message size | +---------------+----------------------+ | 3798185 | 237386.5625 | +---------------+----------------------+
How many miles did the drivers in the driver_log table travel? What was the average miles traveled per day?
mysql> SELECT SUM(miles) AS 'total miles', -> AVG(miles) AS 'average miles/day' -> FROM driver_log; +-------------+-------------------+ | total miles | average miles/day | +-------------+-------------------+ | 2166 | 216.6000 | +-------------+-------------------+
What is the total population of the United States?
mysql> SELECT SUM(pop) FROM states; +-----------+ | SUM(pop) | +-----------+ | 248102973 | +-----------+
(The value represents the population reported for April, 1990. The figure shown here differs from the U.S. population reported by the U.S. Census Bureau, because the states table doesn't contain a count for Washington, D.C.)
SUM( ) and AVG( ) are strictly numeric functions, so they can't be used with strings or temporal values. On the other hand, sometimes you can convert non-numeric values to useful numeric forms. Suppose a table stores TIME values that represent elapsed time:
mysql> SELECT t1 FROM time_val; +----------+ | t1 | +----------+ | 15:00:00 | | 05:01:30 | | 12:30:20 | +----------+
To compute the total elapsed time, use TIME_TO_SEC( ) to convert the values to seconds before summing them. The result also will be in seconds; pass it to SEC_TO_TIME( ) should you wish the sum to be in TIME format:
mysql> SELECT SUM(TIME_TO_SEC(t1)) AS 'total seconds', -> SEC_TO_TIME(SUM(TIME_TO_SEC(t1))) AS 'total time' -> FROM time_val; +---------------+------------+ | total seconds | total time | +---------------+------------+ | 117110 | 32:31:50 | +---------------+------------+
The SUM( ) and AVG( ) functions are especially useful in applications that compute statistics. They're explored further in Chapter 13, along with STD( ), a related function that calculates standard deviations.