定义：

AVG 函数返回数值列的平均值。NULL 值不包括在计算中。

语法：

SELECT AVG(column_name) FROM table_name

 -- 实际操作（计算salary的平均值）：

SELECT avg(salary) AS "平均工资" FROM employee ;

 -- 综合实际操作：

SELECT title,
       COUNT(salary) AS "统计每组数据",
       SUM(salary) AS "统计工资总和",
       MAX(salary) "统计每组最大值",
       MIN(salary) "统计每组最小值",
       AVG(salary) "统计平均值" FROM employee 
       GROUP BY title 
       ORDER BY avg(salary) DESC;