Question

Here is my tables

 Student (sname, sid, gpa, level, deptno) 
 Course (cno, cname, deptno, units) 
 Dept (dname, deptno) 
 Takes (sid, cno) 

Write a SQL query that returns the names (i.e., snames) of students who have taken
more courses outside their department than inside their department.you can
assume that all students in the database have taken at least one course inside their department.

I am not looking for any solutions for this question, but still welcome for any answer.
But I more hoping people can tell me how to generate the steps to write a complicated query like this..

My answer is

 Select S.sname
 From Student S, Course C, Dept D, Takes T
 Where T.cno=C.cno and D.deptno=C.deptno and S.sid = T.sid
 Having COUNT(S.deptno=C.deptno) > COUNT( S.deptno != C.deptno)

I am not sure I can use the count after HAVING in this way or not .
Thanks

Answer 1

Your first attempt, corrected:

 SELECT S.sname
 FROM Student S, Course C, Dept D, Takes T
 WHERE T.cno = C.cno AND D.deptno = C.deptno AND S.sid = T.sid
 GROUP BY S.sid
 HAVING COUNT(CASE WHEN S.deptno =  C.deptno THEN 1 ELSE NULL END) 
      < COUNT(CASE WHEN S.deptno <> C.deptno THEN 1 ELSE NULL END) ;

and converted to SQL-92 syntax:

 SELECT S.sname
 FROM Student S 
   JOIN Takes T   ON S.sid = T.sid
   JOIN Course C  ON T.cno = C.cno
   JOIN Dept D    ON D.deptno = C.deptno
 GROUP BY S.sid        
        , S.sname                  --- this is not needed in SQL-2003
                                   --- but still required by most DBMS     
 HAVING 
        COUNT(CASE WHEN S.deptno =  C.deptno THEN 1 END) 
      < COUNT(CASE WHEN S.deptno <> C.deptno THEN 1 END) ;

                                   --- the ELSE NULL is not needed either
                                   --- it's the implied default