Question

I have basic many to many relation set with tables categories, locations trough categories_locations table. Example:

Categories table

| ID | Name       |
| 1  | Category 1 |
| 2  | Category 2 |
| 3  | Category 3 |

Locations table

| ID | Name       |
| 1  | Location 1 |
| 2  | Location 2 |
| 3  | Location 3 |

Categories_Locations table

| category_id | location_id |
| 1           | 1           |
| 2           | 2           |
| 2           | 3           |
| 3           | 1           |
| 3           | 3           |

How to get all location that belong to category 2 and at the same time also belong to category 3? In above example that would result only to location 3!

Filtering with OR is simple. Just a normal left join where category_id IN (matched categories). But how to filter when I want to get only those relation that belong to category1 and at the same time also to category2 (and so on)?

Answer 1

select 
    Location_ID 
from CategoryLocations
where Category_ID in (2,3)
group by Location_ID
having COUNT(distinct Category_ID) = 2  -- this 2 is the number of items in the IN list above