Oracle SQL plus 11g中的多表联接查询

Question

I have three tables.

1. table SCHOOL : schoolcode(PK), year, schoolname.
2. table ENROLMENT : schoolcode, year, classname, enrol
3. table CLASS : schoolcode, year, classid, rooms

Now, I want to find the list of schools with enrollment in classname - 1 to 4 and number of classrooms used by class 1-4.

I used the following query:

select 
    m.schoolcode, m.schoolname, sum(e.c1+e.c2+e.c3+e.c4), sum(c.rooms) 
from 
    dise2k_enrolment09 e, dise2k_master m, dise2k_clsbycondition 
where 
    m.schoolcode = e.schoolcode 
    and m.schoolcode = c.schoolcode 
    and e.year = '2011-12' and m.year = '2011-12' and c.year = '2011-12' 
    and classid in (1,2,3,4) 
    and e.classname in (1,2,3,4) 
group by 
    m.schoolcode, m.schoolname 

but the result showing is not correct. Enrollment is showing much higher than actual, same in case of classrooms.

Answer 1

Try this:

select m.schoolcode, m.schoolname, sum(e.c1+e.c2+e.c3+e.c4), sum(c.rooms) 
from dise2k_enrolment09 e, dise2k_master m ,dise2k_clsbycondition c
where m.schoolcode=e.schoolcode and m.schoolcode=c.schoolcode and e.year='2011-12' and m.year='2011-12' and c.year='2011-12' 
and c.classid in(1,2,3,4) 
and e.classname = c.classid
group by m.schoolcode, m.schoolname 

The way you have it: and e.classname in(1,2,3,4) is like having an OR operator in your where clause.

(c.classid=1 or c.classid=2 or c.classid=3 or c.classid=4) 
and 
(e.classname=1 or e.classname=2 or e.classname=3 or e.classname=4)

So, c.classid can be "1" and e.classname can be "2" which is wrong

UPDATE I still think the problem is that you don't connect the c.classid with e.classname

Try it like this:

select m.schoolcode, m.schoolname, sum(e.c1+e.c2+e.c3+e.c4), sum(c.rooms) 
from dise2k_enrolment09 e, dise2k_master m ,dise2k_clsbycondition c
where m.schoolcode=e.schoolcode and m.schoolcode=c.schoolcode and e.year='2011-12' and m.year='2011-12' and c.year='2011-12' 
and c.classid in(1,2,3,4) 
and c.classid = decode(e.classname,1,7,2,7,3,8,4,8,5,9,6,9,7,10,8,10)
group by m.schoolcode, m.schoolname 

Answer 2

try this:

 select m.schoolcode, m.schoolname, sum(e.c1+e.c2+e.c3+e.c4), sum(c.rooms) 
from dise2k_enrolment09 e join dise2k_master m
on  m.schoolcode=e.schoolcode 
join dise2k_clsbycondition c
on m.schoolcode=c.schoolcode
where  e.year='2011-12' and m.year='2011-12' and c.year='2011-12' 
and classid in(1,2,3,4) 
and e.classname in(1,2,3,4) 
group by m.schoolcode, m.schoolname