Count number of ratings - mysql

Question

I want to calculate number of every rating group by given date range. I wrote the following query which is working perfect:
SELECT c.day,
(SELECT COUNT(DISTINCT user_id) FROM ratings r WHERE DATE(r.created_at) = c.day AND r.rating = 1 AND r.campaign_id = 2) AS rating1s,
(SELECT COUNT(DISTINCT user_id) FROM ratings r WHERE DATE(r.created_at) = c.day AND r.rating = 2 AND r.campaign_id = 2) AS rating2s,
(SELECT COUNT(DISTINCT user_id) FROM ratings r WHERE DATE(r.created_at) = c.day AND r.rating = 3 AND r.campaign_id = 2) AS rating3s,
(SELECT COUNT(DISTINCT user_id) FROM ratings r WHERE DATE(r.created_at) = c.day AND r.rating = 4 AND r.campaign_id = 2) AS rating4s,
(SELECT COUNT(DISTINCT user_id) FROM ratings r WHERE DATE(r.created_at) = c.day AND r.rating = 5 AND r.campaign_id = 2) AS rating5s
FROM calendar c
WHERE c.day >= '2018-08-01'
GROUP BY c.day
ORDER BY c.day
LIMIT 0, 31
But this is not an optimized way due to 5 sub queries and query is taking almost 2mins on my localhost, how can I optimize this query? The sample output is attached and I need same output.

Answer 1

You can rephrase this as conditional aggregation:
SELECT DATE(r.created_at),
COUNT(DISTINCT CASE WHEN r.rating = 1 THEN r.user_id END) as raging_1,
COUNT(DISTINCT CASE WHEN r.rating = 2 THEN r.user_id END) as raging_2,
COUNT(DISTINCT CASE WHEN r.rating = 3 THEN r.user_id END) as raging_3,
COUNT(DISTINCT CASE WHEN r.rating = 4 THEN r.user_id END) as raging_4,
COUNT(DISTINCT CASE WHEN r.rating = 5 THEN r.user_id END) as raging_5
FROM ratings r
WHERE r.campaign_id = 2 AND
r.created_at >= '2018-08-01'
GROUP BY DATE(r.created_at);
COUNT(DISTINCT) can be expensive. Remove it if you can.
Otherwise, it might be faster to do the DISTINCT once:
SELECT dte,
SUM( r.rating = 1 ) as raging_1,
SUM( r.rating = 2 ) as raging_2,
SUM( r.rating = 3 ) as raging_3,
SUM( r.rating = 4 ) as raging_4,
SUM( r.rating = 5 ) as raging_5
FROM (SELECT DISTINCT user_id, rating, DATE(r.created_at) as dte
FROM ratings r
WHERE r.campaign_id = 2 AND
r.created_at >= '2018-08-01'
) urd
GROUP BY dte;
This returns rows for each day that has at least one rating. If some days would have all zeroes, then you'll need an outer join of some sort. That adds almost nothing to the performance, so it can be tacked on if one of the above solutions works.

Answer 2

Here is a query I made using #Gordon's answer:
SELECT DATE(r.created_at),
COUNT(
DISTINCT
CASE
WHEN r.rating = 1
THEN user_id
ELSE 0
END
) as rating1s,
COUNT(
DISTINCT
CASE
WHEN r.rating = 2
THEN user_id
ELSE 0
END
) as rating2s,
COUNT(
DISTINCT
CASE
WHEN r.rating = 3
THEN user_id
ELSE 0
END
) as rating3s,
COUNT(
DISTINCT
CASE
WHEN r.rating = 4
THEN user_id
ELSE 0
END
) as rating4s,
COUNT(
DISTINCT
CASE
WHEN r.rating = 5
THEN user_id
ELSE 0
END
) as rating5s
FROM ratings r
WHERE r.campaign_id = 2 AND
DATE(r.created_at) >= '2018-08-01'
GROUP BY DATE(r.created_at)
This is still not optimized but much better than my initial solution.