A few tricky SQL statements

Question

I'm trying to do two fairly tricky SQL statements for a class, and been trying for a while and have gotten stuck.
The first one is, "Find Courses where more than half the students got an A"
I've basically been trying to use not exist statements to set it up and then compared the count(sid)/2 > count(sid) query of all A's and am just not getting decent results from it.
The REALLY tricky one is "Find the pairs of students who took exactly the same set of courses"
I can get a cross product from all students with something like this
SELECT s1.sname, s2.sname
FROM lab6_student s1, lab6_student s2
WHERE s1.sname > s2.sname
but after that I can't seem to get it to work properly at all.
Thanks for any pointers/advice!

Answer 1

1.
SQL> WITH T
  2       AS (SELECT 'mathematics' course_name, 'john' student_name, 'A' grade FROM DUAL
  3           UNION ALL
  4           SELECT 'mathematics' course_name, 'john1' student_name, 'A' grade FROM DUAL
  5           UNION ALL
  6           SELECT 'mathematics' course_name, 'john2' student_name, 'A' grade FROM DUAL
  7           UNION ALL
  8           SELECT 'mathematics' course_name, 'john3' student_name, 'B' grade FROM DUAL
  9           UNION ALL
10           SELECT 'physics' course_name, 'john' student_name, 'A' grade FROM DUAL
11           UNION ALL
12           SELECT 'physics' course_name, 'john1' student_name, 'B' grade FROM DUAL
13           UNION ALL
14           SELECT 'physics' course_name, 'john2' student_name, 'B' grade FROM DUAL
15           UNION ALL
16           SELECT 'physics' course_name, 'john3' student_name, 'B' grade FROM DUAL)
17  SELECT   *
18    FROM   T;
COURSE_NAME STUDE G
mathematics john  A
mathematics john1 A
mathematics john2 A
mathematics john3 B
physics     john  A
physics     john1 B
physics     john2 B
physics     john3 B
8 rows selected.
SQL> WITH T
  2       AS (SELECT 'mathematics' course_name, 'john' student_name, 'A' grade FROM DUAL
  3           UNION ALL
  4           SELECT 'mathematics' course_name, 'john1' student_name, 'A' grade FROM DUAL
  5           UNION ALL
  6           SELECT 'mathematics' course_name, 'john2' student_name, 'A' grade FROM DUAL
  7           UNION ALL
  8           SELECT 'mathematics' course_name, 'john3' student_name, 'B' grade FROM DUAL
  9           UNION ALL
10           SELECT 'physics' course_name, 'john' student_name, 'A' grade FROM DUAL
11           UNION ALL
12           SELECT 'physics' course_name, 'john1' student_name, 'B' grade FROM DUAL
13           UNION ALL
14           SELECT 'physics' course_name, 'john2' student_name, 'B' grade FROM DUAL
15           UNION ALL
16           SELECT 'physics' course_name, 'john3' student_name, 'B' grade FROM DUAL)
17  SELECT   *
18    FROM   (SELECT     course_name, COUNT (*) total_cnt, SUM (CASE WHEN grade = 'A' THEN 1 ELSE 0 END) grade_cnt
19                FROM   T
20            GROUP BY   course_name)
21   WHERE   grade_cnt > total_cnt / 2;
COURSE_NAME  TOTAL_CNT  GRADE_CNT
mathematics          4          3
SQL>
2.
SQL> WITH T
  2       AS (SELECT '1' course_id, 'john' student_name FROM DUAL
  3           UNION ALL
  4           SELECT '2' course_id, 'john' student_name FROM DUAL
  5           UNION ALL
  6           SELECT '1' course_id, 'peter' student_name FROM DUAL
  7           UNION ALL
  8           SELECT '2' course_id, 'peter' student_name FROM DUAL
  9           UNION ALL
10           SELECT '3' course_id, 'king' student_name FROM DUAL
11           UNION ALL
12           SELECT '4' course_id, 'king' student_name FROM DUAL)
13  SELECT   *
14    FROM t;
C STUDE
1 john
2 john
1 peter
2 peter
3 king
4 king
6 rows selected.
SQL> WITH T
  2       AS (SELECT '1' course_id, 'john' student_name FROM DUAL
  3           UNION ALL
  4           SELECT '2' course_id, 'john' student_name FROM DUAL
  5           UNION ALL
  6           SELECT '1' course_id, 'peter' student_name FROM DUAL
  7           UNION ALL
  8           SELECT '2' course_id, 'peter' student_name FROM DUAL
  9           UNION ALL
10           SELECT '3' course_id, 'king' student_name FROM DUAL
11           UNION ALL
12           SELECT '4' course_id, 'king' student_name FROM DUAL),
13       r
14       AS (SELECT     student_name, SUM (POWER ( 2, course_id)) val
15               FROM   T
16           GROUP BY   student_name)
17  SELECT   DISTINCT student_name NAME
18    FROM   r
19   WHERE   EXISTS
20              (SELECT     'x'
21                   FROM   r r1
22                  WHERE   r1.val = r.val
23               GROUP BY   val
24                 HAVING   COUNT (*) > 1);
NAME
john
peterEdited by: G. on Feb 24, 2011 2:09 PM