这两个查询有什么区别？

Question

SELECT DISTINCT S.sname
FROM   student S,
       enrolled E1,
       enrolled E2,
       class C1,
       class C2
WHERE  S.snum = E1.snum
       AND S.snum = E2.snum
       AND E1.cname <> E2.cname
       AND E1.cname = C1.NAME
       AND E2.cname = C2.NAME
       AND C1.meets_at = C2.meets_at;  

and

SELECT DISTINCT S.sname
FROM   student S
WHERE  S.snum IN (SELECT E1.snum
                  FROM   enrolled E1,
                         enrolled E2,
                         class C1,
                         class C2
                  WHERE  E1.snum = E2.snum
                         AND E1.cname <> E2.cname
                         AND C1.meets_at = C2.meets_at);  

both the queries are executing and showing output The first query is showing output as empty set while the second query is showing some non empty output

I thought both the queries will give me the same and correct output but only second query is showing correct result. Can anyone please explain whats the difference between both the queries? Thanks in advance.

Answer 1

There is no join relationship specified between the "Es" and the "Cs" in the second query so that's going to be a cross join (everything matches with everything). You have this in the first query and nothing comparable in the second:

       AND E1.cname = C1.NAME
       AND E2.cname = C2.NAME