在mySQL中求平均值

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

如果你想得到小组的平均值,你可能应该这样做:

SELECT AVG(mpg) from car;

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

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

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

WITH ROLLUP -修饰符将在结果集中添加一行,其中 persion_id NULL 和 mpg_avg 是整个结果集的平均值(MySQL>=4.1.1):

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