Question

Ok, i’ve been trying to solve this for about 2 hours now… Please advise:

Tables:

PROFILE [id (int), name (varchar), ...] SKILL   [id (int), id_profile (int), id_app (int), lvl (int), ...] APP     [id (int), ...] 

The lvl can basically go from 0 to 3.

I’m trying to get this particular stat: ‘What is the percentage of apps that is covered by at least two people having a skill of 2 or higher?’

Thanks a lot

Answer 1

SELECT AVG(covered) FROM (   SELECT CASE WHEN COUNT(*) >= 2 THEN 1 ELSE 0 END AS covered   FROM app a   LEFT JOIN skill s ON (s.id_app = a.id AND s.lvl >= 2)   GROUP BY a.id ) 

More efficient way for MySQL:

SELECT AVG        (          IFNULL          (            (            SELECT 1            FROM skill s            WHERE s.id_app = a.id            AND s.lvl >= 2            LIMIT 1, 1            ), 0          )        ) FROM app a 

This will stop counting as soon as it finds the second skilled person for each app.

Efficient if you have a few app‘s but lots of person‘s.