MySQL: 4 Table "has-many-through" Join?

Question

Let's say I have the following 4 tables (for examples' sake): Owners, Trucks, Boxes, Apples.

An owner can have many trucks, a truck can have many boxes and a box can have many apples.

Owners have an id. Trucks have an id and owner_id. Boxes have an id and truck_id. Apples have an id and box_id.

Let's say I want to get all the apples "owned" by an owner with id = 34. So I want to get all the boxes that are in boxes that are in trucks that owner 34 owns.

There is a "hierarchy" if you will of 4 tables that each only has reference to its direct "parent". How can I quickly filter boxes while satisfying conditions across the other 3 tables?

I hope that made sense somewhat.

Thanks.

Answer 1

select a.* 
from Trucks t
inner join Boxes b on t.id = b.truck_id
inner join Apples a on b.id = a.box_id
where t.owner_id = 34

Answer 2

You just start at the "top" (owners) and keep joining until you get where you want:

SELECT a.*
FROM Owners o
INNER JOIN Trucks t ON t.owner_id = o.id
INNER JOIN Boxes b on b.truck_id = t.id
INNER JOIN Apples a on a.box_id = b.id
WHERE o.id = ?

If queries like that are needed often and you are working with very large data sets, sometimes it makes sense to denormalize the data a bit as well. For example by adding the owner_id to the apples table. It makes inserting/updating the data a bit more difficult, but can make queries easier.

Answer 3

    SELECT a.*
      FROM Apples a
INNER JOIN Boxes b ON b.id = a.box_id
INNER JOIN Trucks t ON t.id = b.truck_id
INNER JOIN Owners o ON o.id = t.owner_id
     WHERE o.id = 34

You can simplify this somewhat by leaving out the join to owners and just selecting where t.owner_id = 34 if you don't need any information about the owner later.