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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.

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References

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MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois
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