Finding users with at least one of every item - mysql

Question

For example, I have the following table called, Information
user_id | item
-------------------------
45 | camera
36 | smartphone
23 | camera
1 | glucose monitor
3 | smartwatch
2 | smartphone
7 | smartphone
2 | camera
2 | glucose monitor
2 | smartwatch
How can I check which user_id has at least one of every item?
The following items will not be static and may be different everytime. However in this example there are 4 unique items: camera, smartphone, smartwatch, glucose monitor
Expected Result:
Because user_id : 2 has at least one of every item, the result will be:
user_id
2
Here is what I attempted at so far, however if the list of items changes from 4 unique items to 3 unique items, I don't think it works anymore.
SELECT *
FROM Information
GROUP BY Information.user_id
having count(DISTINCT item) >= 4

Answer 1

One approach would be to aggregate by user_id, and then assert that the distinct item_id count matches the total distinct item_id count from the entire table.
SELECT
user_id
FROM Information
GROUP BY
user_id
HAVING
COUNT(DISTINCT item_id) = (SELECT COUNT(DISTINCT item_id) FROM Information);

Answer 2

You can try to use self-join by count and total count
SELECT t1.user_id
FROM (
SELECT user_id,COUNT(DISTINCT item) cnt
FROM T
GROUP BY user_id
) t1 JOIN (SELECT COUNT(DISTINCT item) cnt FROM T) t2
WHERE t1.cnt = t2.cnt
or exists
Query 1:
SELECT t1.user_id
FROM (
SELECT user_id,COUNT(DISTINCT item) cnt
FROM T
GROUP BY user_id
) t1
WHERE exists(
SELECT 1
FROM T tt
HAVING COUNT(DISTINCT tt.item) = t1.cnt
)
Results:
| user_id |
|---------|
| 2 |

Answer 3

One more way of solving this problem is by using CTE and dense_rank function.
This also gives better performance on MySQL. The Dense_Rank function ranks every item among users. I count the number of distinct items and say pick the users who have the maximum number of distinct items.
With Main as (
Select user_id
,item
,Dense_Rank () over (
Partition by user_id
Order by item
) as Dense_item
From information
)
Select
user_id
From Main
Where
Dense_item = (
Select
Count(Distinct item)
from
information);