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Question

I have an association called Includes between Parkings and Cars .

This is a one-to-many association that is partial both sides, because a parking can contain zero or more cars and a car can be contained by zero or one parking.

I'd add a FOREIGN KEY related to a parking in the Cars table, but it's not true that a car is always inside a parking.

How can this situation be managed?

Answer 1

So, the car has a column saying where it is currently parked. Or NULL if it is roaming the roads?

And the parking lot (car park) can have lots of cars? But it needs no column(s) to indicate such. The information can be had by a JOIN with the other table.

A FOREIGN KEY is 2 things:

• An integrity check. You wan't want a car parked in a nonexistent lot .
• An INDEX to make the above JOIN more efficient.

We can't judge the indexes needed without seeing the SELECTs (and other queries).

To find all the cars in one lot:

SELECT *
    FROM cars
    WHERE lot = 123 

That needs INDEX(lot) (or the FK) in cars .

To find where one car is:

SELECT lot FROM cars WHERE id = 456;

Presumable cars has PRIMARY KEY(id) .