Question

So I’ve got some data. There are entities. Entities have an arbitrary number of items. Items can be one of a defined set of types. An entity can have more than one item of a given type. I can get a list of items that an entity has. What I want is to get a list of types that an entity doesn’t have an item for.

Here’s my schema:

entities
id name
1  Bob
2  Alice

item_types
id      name 
1       red
2       yellow
3       green
4       blue
5       orange

items
entity_id item_type_id name
1         1            apple
1         2            banana
1         3            lime
1         3            tree
2         3            money
2         5            traffic cone

I would like to query Bob’s id (1) and get this list:

4   blue
5   orange

And query Alice’s id (2) and get:

1   red
2   yellow
4   blue

It’s probably starting me in the face. I’m gonna keep working on it but I bet you SO peeps beat me to it. Thank you kindly for your time.

Answer 1

select id, name
from item_types
where id not in
    (select i.item_type_id
    from items i
    inner join entities e
        on e.id = t.entity_id
    where e.Name = 'Bob')

or (sometimes faster, but optimizers are getting better all the time):

select disctinct t.id, t.name
from item_types t
left outer join items i
    on i.item_type_id = t.id
left outer join entities e
    on e.id = i.entity_id
    and e.Name = 'Bob'
where e.id is null