Mysql错误：Every derived table must have its own alias

求比每门课程平均分低的学生

问题描述：

Mysql错误：Every derived table must have its own alias 每个派生的表必须有它自己的别名

 

错误SQL：

[Err] 1248 - Every derived table must have its own alias

[SQL] SELECT student,course,score FROM score as s, 

(SELECT course as a_course,AVG(score) as a_score FROM score GROUP BY course) 

where s.course = a_course and s.score<a_score

正确SQL：

SELECT student,course,score FROM score as s, 

(SELECT course as a_course,AVG(score) as a_score FROM score GROUP BY course) AS avg_score

where s.course = a_course and s.score<a_score

TIP:

把内层查询结果当作外层查询的比较条件

mysql常用查询：groupby,子查询,having,where