I am trying to select from a table with the average of columns from other tables from other tables; The schema is as follows.
Students ( sid
, firstname
, l astname
, status
, gpa
, email
)
Enrollments ( sid
, classid
, lgrade
)
Grades ( lgrade
, ngrade
)
And the erroneous query is,
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。
There are several issues:
avg()
function. GROUP BY
clause is required somewhere. All in all, I think the aggregate should be inside the parentheses.
Try:
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;
Other notes: e.lgrade is not null
is not needed since the condition g.lgrade = e.lgrade
already ensures it won't be null.
Last, I encourage you to learn ANSI join syntax. You'll thank me later. Seriously, using old-style joins is awful.
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;
In fact, I have a suspicion that simply rewriting the query this way will help expose what's wrong with it--it looks to me like maybe the grades
and enrollments
tables need another join condition?
Two INNER JOIN
s without sub-queries
- please try.
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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