Summarizing with SUM( ) and AVG( )

Summarizing with SUM() and AVG()

7.4.1 Problem

You need to add up a set of numbers or find their average.

7.4.2 Solution

Use the SUM( ) or AVG( ) functions.

7.4.3 Discussion

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 |
+---------------+------------+

7.4.4 See Also

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.