MySql How Return All Items Which Contain All Selected Features

Question

assigned_features

feature_id      |   item_id
-----------------------------------------------
1               |   1
2               |   1
3               |   1
1               |   2
2               |   2
1               |   3

Using mysql, I'm trying to return all items which contain all selected features.

For example, I wish to return items where feature_id = 1 and 2. This should return item_id 1 and 2.

Alternatively, if I only wish to retrieve items where feature_id = 1,2 and 3, the result should yield item 1.

I don't really know where to begin.

Thank you!

Answer 1

You can do aggregation :

select item_id
from assigned_features af
where feature_id in (1,2)
group by item_id
having count(distinct feature_id) = 2;

In similarly you can just increment the count for second example :

select item_id
from assigned_features af
where feature_id in (1,2,3)
group by item_id
having count(distinct feature_id) = 3;

However, DISTINCT is redundant here if item_id doesn't have a duplicate feature_id s then you can use count(*) = 2 or count(*) = 3 instead.