7.4. 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.