MySQL: limit query or subquery (in left/inner join?) - mysql

Question

Apologise in advance, I'm novice in (My)SQL - this should be an easy question for expert DBAs - but I don't even know where to start finding a solution at all. I'm not even sure if I applied LEFT JOIN in the correct way below.
My (DB) structure is quite simple:
I have testsuites, and several testcases are linked to each testsuite ("logical entities")
During testcase kick-off, I'm creating an entry for each testsuite in the testsuiteinstance table - and one entry in testcaseinstance for each testcase.
My goal is to fetch the last 10 testcaseinstances of all testcases belonging to a certain testsuite
This is the query I use to fetch all testcaseinstances:
SELECT * FROM testcaseinstance AS tcinst
LEFT JOIN testsuiteinstance tsinst ON tsinst.id=tcinst.testsuiteinstance_id
LEFT JOIN testsuite ts ON ts.id=tsinst.testsuite_id
WHERE ts.id = 349 ORDER BY tcinst.id DESC;
So, let's say I have two testcases in a testsuite and both testcase was executed 100 times each. This query gives me 200 rows. If I put "LIMIT 10" at the end, I will only get the last 10 rows for one testcase type, but I want 20 rows (the last 10-10 belonging to the two testcases)
I'd appreciate some description beside the solution query or a pointer to a "tutorial" I can start looking at related to the topic (whatever would that be :D)
Thanks in advance!

Answer 1

Here's one approach; consider this (slightly contrived) example...
SELECT * FROM ints;
+---+
| i |
+---+
| 0 |
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
+---+
Let's say we want to return the top 3 even numbers and the top 3 odd numbers from this list. Ignoring for the moment that there's another, simpler, solution to this particular example we can instead do something like this...
SELECT x.*
, COUNT(*) rank
FROM ints x
JOIN ints y
ON MOD(y.i,2) = MOD(x.i,2)
AND y.i >= x.i
GROUP
BY i
ORDER
BY MOD(x.i,2) DESC
, x.i DESC;
+---+------+
| i | rank |
+---+------+
| 9 | 1 |
| 7 | 2 |
| 5 | 3 |
| 3 | 4 |
| 1 | 5 |
| 8 | 1 |
| 6 | 2 |
| 4 | 3 |
| 2 | 4 |
| 0 | 5 |
+---+------+
From here, the process of grabbing just the top 3 from each group becomes trivial...
SELECT x.*
, COUNT(*) rank
FROM ints x
JOIN ints y
ON MOD(y.i,2) = MOD(x.i,2)
AND y.i >= x.i
GROUP
BY i
HAVING rank <=3
ORDER
BY MOD(x.i,2),x.i DESC;
+---+------+
| i | rank |
+---+------+
| 8 | 1 |
| 6 | 2 |
| 4 | 3 |
| 9 | 1 |
| 7 | 2 |
| 5 | 3 |
+---+------+
...and this can be simplified to...
SELECT x.*
FROM ints x
JOIN ints y
ON MOD(y.i,2) = MOD(x.i,2)
AND y.i >= x.i
GROUP
BY i
HAVING COUNT(*) <=3;
+---+
| i |
+---+
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
+---+