Question

我有一张桌子

车

 id |  person_id  |  mpg
------------------------  
 4  |     1       |  50
 5  |     1       |  15
 6  |     2       |  10
 7  |     2       |  28
 8  |     3       |  33

我需要得到每个人mpg的平均值，然后是该组的平均值。

person 1 avg = (50 + 15) / 2 = 32.5
person 2 avg = (10 + 28) / 2 = 19
person 3 avg = 33

group average = 32.5 + 19 + 33 / 3 = 28.1

是否有可以满足我需要的查询？

Answer 1

SELECT person_id, AVG(mpg) from car group by person_id;

如果你想获得该组的平均值，你应该这样做：

SELECT AVG(mpg) from car;

除非你真的想平均平均值，这对我来说似乎有点可疑：

SELECT AVG(average) from (SELECT person_id, AVG(mpg) as average from car group by person_id);

Answer 2

你无法在1个查询中解决这个问题，但你必须使用2个查询或1个查询来解决代码中的总体平均值

select person, avg(mpg) from cat group by person

Answer 3

SELECT person_id, AVG(mpg) AS mpg_avg FROM car GROUP BY person_id WITH ROLLUP

WITH ROLLUP - 修饰符会在persion_id为NULL的结果集中添加一行，mpg_avg是整个结果集的平均值（MySQL＆gt; = 4.1.1）：

 person_id | mpg 
------------------
         1 | 32.5 
         2 | 19.0
         3 | 33.0
      NULL | 27.2

平均mySQL中的平均值

3 个答案: