How to write SQL subqueries with count? - sql

Question

I'm using mysql. I want to count the minimum, maximum, and average number of different tags per movie from one table.
Exclude the duplicate:
same tag given by the same user to the same movie
same tag given by different users to the same movie
Example: table 'tags'
userId
movieId
tag
1
1
crime
1
2
dark
1
2
dark
2
2
greed
2
2
dark
3
3
music
3
3
dance
3
3
quirky
4
3
dance
4
3
quirky
Expect result:
movieId
Min_Tag
Max_Tag
Avg_Tag
1
1
1
1
2
1
2
0.66...
3
1
2
0.6
I try to write query like below, but it shows an error.
SELECT
DISTINCT movieId,
MIN(COUNT(DISTINCT tag) AS Min_Tag,
MAX(COUNT(DISTINCT tag) AS Max_Tag,
AVG(COUNT(DISTINCT tag) AS Avg_Tag,
FROM (
SELECT userId,movieId,tag
FROM tags
GROUP BY userId, movieId, tag
) AS non_dup
GROUP BY movieId;

Answer 1

You have to rewrite your query
First you need the count per tag and movie
and fromm that you can calculate min max and avg
SELECT
movieId,
MIN(count_tag) AS Min_Tag,
MAX(count_tag) AS Max_Tag,
AVG(count_tag) AS Avg_Tag
FROM
(SELECT movieId,tag, count(*) count_tag
FROM tags
GROUP BY movieId, tag) non_dup
GROUP BY movieId
movieId
Min_Tag
Max_Tag
Avg_Tag
1
1
1
1.0000
2
1
3
2.0000
3
1
2
1.6667
fiddle