Question

I found it hard to find a fitting title. For simplicity let’s say I have the following table:

cook_id cook_rating
1       2
1       1
1       3
1       4
1       2
1       2
1       1
1       3
1       5
1       4
2       5
2       2

Now I would like to get an output of ‘good’ cooks. A good cook is someone who has a rating of at least 70% of 1, 2 or 3, but not 4 or 5.

So in my example table, the cook with id 1 has a total of 10 ratings, 7 of which have type 1, 2 and 3. Only three have type 4 or 5. Therefore the cook with id 1 would be a ‘good’ cook, and the output should be the cook’s id with the number of good ratings.

cook_id cook_rating
1       7

The cook with id 2, however, doesn’t satisfy my condition, therefore should not be listed at all.

select cook_id, count(cook_rating) - sum(case when cook_rating = 4 OR cook_rating = 5 then 1 else 0 end) as numberOfGoodRatings from cook
where cook_rating in (1,2,3,4,5)
group by cook_id
order by numberOfGoodRatings desc

However, this doesn’t take into account the fact that there might be more 4 or 5 than good ratings, resulting in negative outputs. Plus, the requirement of at least 70% is not included.

Answer 1

You can get this with a comparison in your HAVING clause. If you must have just the two columns in the result set, this can be wrapped as a sub-select select cook_id, positive_ratings FROM (...)

SELECT 
  cook_id, 
  count(cook_rating < 4 OR cook_rating IS NULL) as positive_ratings, 
  count(*) as total_ratings
FROM cook
GROUP BY cook_id
HAVING (positive_ratings / total_ratings) >= 0.70
ORDER BY positive_ratings DESC

Edit Note that count(cook_rating < 4) is intended to only count rows where the rating is less than 4. The MySQL documentation says that count will only count non-null rows. I haven’t tested this to see if it equates FALSE with NULL but I would be surprised it it doesn’t. Worst case scenario we would need to wrap that in an IF(cook_rating < 4, 1,NULL).