如何在四个表上应用左外部联接条件？

Question

i was trying to apply joins on 4 tables. but i could not find proper result for that. i have 4 tables like 1.students,2.college,3.locations,4.departments. so i have same column sid in all tables which can be used to join conditions. i want all matched rows from four tables as mentioned columns in select statement below and unmatched rows in left table which is left outer join work.

i have tried this syntax.

select 
  students.sname,
  college.cname,
  locations.loc,
  department.dept 
from students, college, locaions, departments 
where student.sid=college.sid(+) 
and college.sid=locations.sid(+) 
and locations.sid=department.sid(+);

is this right ?

Answer 1

This is an old-fashioned way of outer-joining in an Oracle database. For Oracle this statement is correct; in other DBMS it is invalid.

Anyway, nowadays (as of Oracle 9i; in other DBMS much longer) you should use standard SQL joins instead.

select 
  s.sname,
  c.cname,
  l.loc,
  d.dept 
from students s
left outer join college c on c.sid = s.sid
left outer join locations l on l.sid = c.sid
left outer join departments d on d.sid = l.sid;

Answer 2

Given what you've shown it would appear that what you really want is

select s.sname,
       c.cname,
       l.loc,
       d.dept 
from students s
LEFT OUTER JOIN college c
  ON c.SID = s.SID
LEFT OUTER JOIN locations l
  ON l.SID = s.SID
LEFT OUTER JOIN departments d 
  ON d.SID = s.SID

The issue in your original query is that because an OUTER JOIN is an optional join, you can end up with NULL values being returned in one of the join fields, which then prevents any of the downstream values being joined. I agree with @ThorstenKettner who observes in a comment that SID is apparently a "student ID", but it's not reasonable or appropriate to have a "student ID" field on tables named COLLEGE, LOCATIONS, or DEPARTMENTS. Perhaps you need to update your design to allow any number of students to be associated with one of these entities, perhaps using a "join" table.

Best of luck.