AVG() 函数返回数值列的平均值。
SELECT AVG(column_name) FROM table_name
在本教程中，我们将使用 web3.xin 样本数据库。
下面是选自 "access_log" 表的数据：
+-----+---------+-------+------------+ | aid | site_id | count | date | +-----+---------+-------+------------+ | 1 | 1 | 45 | 2016-05-10 | | 2 | 3 | 100 | 2016-05-13 | | 3 | 1 | 230 | 2016-05-14 | | 4 | 2 | 10 | 2016-05-14 | | 5 | 5 | 205 | 2016-05-14 | | 6 | 4 | 13 | 2016-05-15 | | 7 | 3 | 220 | 2016-05-15 | | 8 | 5 | 545 | 2016-05-16 | | 9 | 3 | 201 | 2016-05-17 | +-----+---------+-------+------------+
下面的 SQL 语句从 "access_log" 表的 "count" 列获取平均值：
SELECT AVG(count) AS CountAverage FROM access_log;执行以上 SQL 输出结果如下：
mysql> select avg(count) as countaverage from access_log; +--------------+ | countaverage | +--------------+ | 174.3333 | +--------------+ 1 row in set (0.05 sec)下面的 SQL 语句选择访问量高于平均访问量的 "site_id" 和 "count"：
SELECT site_id, count FROM access_log WHERE count > (SELECT AVG(count) FROM access_log);执行以上 SQL 输出结果如下：
mysql> select site_id, count from access_log where count > (select AVG(count) from access_log); +---------+-------+ | site_id | count | +---------+-------+ | 1 | 230 | | 5 | 205 | | 3 | 220 | | 5 | 545 | | 3 | 201 | +---------+-------+ 5 rows in set (0.00 sec)
