mysql中出现 Unknown column ‘xxx‘ in ‘having clause‘

mysql中出现 Unknown column ‘xxx‘ in ‘having clause‘

这是因为在使用group by分组时，后面如果需要再加一个having进行判断，则所判断的字段需要在select后面出现

如：

select c.studentNo, c.name, count(coursename)
from courses a, scores b, students c
where a.courseNo = b.courseNo
and b.studentNo = c.studentNo
GROUP BY c.studentNo
having count(coursename) = 5

如果不出现，则会报错

 

 *代码只是做演示，结果是有问题的