Question

I have a the following tables

• students(sid, sname, age)
• course(cid, cname, duration)
• enroll(sid, cid, date)

To find all sid’s who had taken the courses which sid = 1 has taken.

select sid from enroll where cid in (select cid from enroll where sid=1) 
group by sid having count(*)=(select count(*) from enroll where sid=1) 
minus (select sid from student where sid=1);

1. What does the first count(*) refers to?
2. Is there a simpler version of the query?

Answer 1

The first COUNT(*) refers to the count of the rows belonging to a particular sid.

Reformatted:

SELECT sid
  FROM enroll
 WHERE cid IN (SELECT cid FROM enroll WHERE sid=1) 
 GROUP by sid
HAVING COUNT(*) = (SELECT COUNT(*) FROM enroll WHERE sid=1) 
MINUS (SELECT sid FROM student WHERE sid=1);

You can certainly avoid the MINUS by using:

SELECT sid
  FROM enroll
 WHERE cid IN (SELECT cid FROM enroll WHERE sid=1)
   AND sid != 1
 GROUP by sid
HAVING COUNT(*) = (SELECT COUNT(*) FROM enroll WHERE sid=1)