[英]SQL Expression missing error
我试图从一个表中选择其他表中其他表的列的平均值; 架构如下。
学生 ( sid
, firstname
,l astname
, status
, gpa
, email
)
报名 ( sid
, classid
, lgrade
)
等级 ( lgrade
, ngrade
)
而错误的查询是,
select sid, lastname,
avg( select g.ngrade from grades g, enrollments e
where e.sid = sid and g.lgrade = e.lgrade and e.lgrade is not null
and g.ngrade is not null) as cgpa
from students
order by cgpa asc;
从子查询中取出分号。
有几个问题:
avg()
函数的括号。 GROUP BY
子句。 总而言之,我认为聚合应该在括号内。
尝试:
select
sid,
lastname,
(
select avg(g.ngrade)
from grades g, enrollments e
where e.sid = sid and g.lgrade = e.lgrade
and g.ngrade is not null
) as cgpa
from students
order by cgpa asc;
其他说明: e.lgrade is not null
没有必要的,因为条件g.lgrade = e.lgrade
已经确保了它不会是空。
最后,我鼓励您学习ANSI连接语法。 你以后会感谢我的。 说真的,使用旧式连接很糟糕。
select
s.sid,
s.lastname,
(
select avg(g.ngrade)
from
grades g
inner join enrollments e
on g.lgrade = e.lgrade
where
g.ngrade is not null
and s.sid = g.sid
) as cgpa
from students s
order by cgpa asc;
事实上,我怀疑只是以这种方式重写查询将有助于揭示它的错误 - 它在我看来可能是grades
和enrollments
需要另一个连接条件?
两个INNER JOIN
没有sub-queries
- 请尝试。
SELECT s.sid, s.lastname, avg(g.ngrade) cgpa
FROM Students s
JOIN Enrollments e ON s.sid = e.sid
JOIN Grades g ON e.lgrade=g.lgrade
GROUP BY s.sid, s.lastname
ORDER BY 3;
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