常用聚合函数

SELECT MAX(t.salary) AS 最大值,
       MIN(t.salary) AS 最小值,
       AVG(t.salary) AS 平均值,
       SUM(t.salary) AS 和值,
       COUNT(t.salary) AS 计数,
       STDDEV(t.salary) AS 标准差,
       VARIANCE(t.salary) AS 方差
  FROM temp t;

其中count可以配合having来排查重复数据，例如：

SELECT t.department, COUNT(1)
  FROM temp t
 GROUP BY t.department
HAVING COUNT(1) > 1;