ERROR 1054 (42S22): Unknown colu

环境：
mysql 版本5
错误：

select * from students as stu ,subjects as sub  
left join examinations as ex 
on stu.student_id=ex.student_id
and sub.subject_name=ex.subject_name;

ERROR 1054 (42S22): Unknown column 'stu.student_id' in 'on clause'

解决办法：
需要把联合的表用括号括起来，并且括起来之后不能取别名:
正确写法 ：

select * from (students as stu ,subjects as sub ) 
left join examinations as ex 
on stu.student_id=ex.student_id
and sub.subject_name=ex.subject_name;

注意：括起来之后也不要起别名，不然也会报错