Question

I have two tables, entities and components, each with a name and unique ID. There is a third table mapping the many-to-many relationship between these two IDs called construction, containing the two columns, entity_ID and component_ID. An entity is composed of some number of components. So if the construction table looks like this:

+----------+---------------+
|entity_id |  component_id |
+----------+---------------+
|        1 |             1 | 
|        1 |             2 | 
|        2 |             2 | 
|        2 |             6 | 
+----------+---------------+

Entity 1 is made out of components 1,2, while entity 2 is made out of 2 and 6. The application I’m working on also has an available table containing the IDs of all available components. My question is this, how do I query the database to return the entities constructed strictly of components in the available table? In the given example, if 1,2 and 6 are all in the available table, then entities 1 and 2 should be returned. Otherwise, if 2 isn’t in the available table (but 1 and 6 are) then nothing is returned. I’m new to MySQL so if you could explain the logic as well that would be great.

Answer 1

I tested @Abdullah’s subquery and indeed got the opposite of what I wanted. After a little testing I ended up with the nested queries that give me what I was looking for:

select distinct entity_id from construction 
where entity_id not in (
    select entity_id from construction c left outer join available a on a.id = c.component_id 
    where a.id is null 
);