Question

I have 2 tables, #1 called TESTS, #2 RESULTS

#1 TESTS (TESTID, LEVELID)
#2 RESULTS (USERID, TESTID, SCORE) 

I would like to get TWO COUNTS, #1 total tests for a level, and #2, total tests from RESULTS where score = 100 GROUP BY LEVELID

I have 15 levels, so I basically want to get results something like this

Level 1 - 15 tests, 3 perfect
Level 2 - 25 tests, 5 perfect
....
....
Level 15 - 75 tests, 8 perfect

How can I do this using 1 SQL statement and is that the best way to do it?

Thanks.

Answer 1

It’s a well-posed question and I think the right one to ask in this situation.

First do a join for tests and results:

 SELECT
    t.level,
    SUM(CASE WHEN r.score = 100 THEN 1 ELSE 0 END) AS perfectcount
 FROM tests t
 JOIN results r
    ON t.testid = r.testid
 GROUP BY t.testid

Then extend it with another join on tests to get the level sum:

 SELECT t.level,
     COUNT(*) AS levelcount,
     t0.perfectcount
 FROM tests AS t
 JOIN {
     SELECT
        t.level,
        SUM(CASE WHEN r.score = 100 THEN 1 ELSE 0 END) AS perfectcount
     FROM tests t
     JOIN results r
        ON t.testid = r.testid
     GROUP BY t.testid
) AS t0
    ON t.level = t0.level
GROUP BY t.level