3 table join counting nulls

Question

I have a situation where I have three tables; dealership, car and warehouse. Dealership is related to car and car is related to warehouse. How can a mysql query be constructed to determine the sum of the number of vehicles in the warehouses that were not sent to a dealer?

Here are the data tables:

Dealer
Id   |   name     |                           
_____|____________|
D1   |  Dealer_1  |
D2   |  Dealer_2  |
D3   |  Dealer_3  |
D4   |  Dealer_4  |


Car
Id   |    name   | Dealer_id |
_____|___________|___________|
C1   |  auto_1|  |   D4      |
C2   |  auto_2   |   D4      |
C3   |  auto_3   |   null    |
C4   |  auto_4   |   D1      |
C5   |  auto_5   |   null    |
C6   |  auto_6   |   D3      |


Warehouse
Id   |    name   | Car_id    | vehical_count |
_____|___________|___________|_______________|
W1   | storage_1 |   C2      |   22          |
W2   | storage_2 |   C3      |   43          |
W3   | storage_3 |   C5      |   18          |
W4   | storage_4 |   C6      |   15          |
W5   | storage_5 |   C1      |    8          |
W6   | storage_6 |   null    |    2          |
W7   | storage_7 |   C3      |   37          |
W8   | storage_8 |   null    |   10          |

Thanks!

Answer 1

This should give you warehouse cars not in the car table or in the car table with no dealer assigned.

SELECT
    SUM(w.vehical_count)
FROM
    Warehouse w
    LEFT OUTER JOIN Car c ON c.Id=w.Car_id
WHERE
    c.Dealer_id IS NULL