大家好，又见面了，我是你们的朋友全栈君。

聚合函数

计数类型（count）

SELECT  COUNT(*)
FROM    (
            SELECT  1 AS num
            UNION ALL
            SELECT  1 AS num
            UNION ALL
            SELECT  2 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

SELECT  COUNT(num)
FROM    (
            SELECT  1 AS num
            UNION ALL
            SELECT  1 AS num
            UNION ALL
            SELECT  2 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

两个结果分别为 4 和 3

count(*) 和 count(column) 的区别可以看之前写的这个文章

同时 sum(column) 也会直接忽略 null 值

数学函数（方差:var_pop 标准差:stddev 等）

SELECT  var_pop(num)
FROM    (
            SELECT  3 AS num
            UNION ALL
            SELECT  6 AS num
            UNION ALL
            SELECT  9 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

SELECT  stddev(num)
FROM    (
            SELECT  3 AS num
            UNION ALL
            SELECT  6 AS num
            UNION ALL
            SELECT  9 AS num
            UNION ALL
            SELECT  NULL AS num
        ) 
;

两个结果分别为 6.0 和 2.449489742783178

[3 6 9]  的方差为 6 ，标准差为 6 的平方根

两个结果对比可以发现，也是直接忽略 null 值的，并不会把它当做 0 处理