Simple database design question

Hi all,
I have 2 tables, "sims" (as in mobile phone sim cards) and "phones" (as
in mobile phones). I can add any number of sim cards to the "sims"
table and any number of mobile phones to the "phones" table. Simple enough.

Rule 1: A sim card can either be in a phone, or not in a phone. It
cannot be in more than one phone.

Rule 2: A phone can either have one sim inserted, or no sim inserted.

So the relationship between the phone and sim is 0:1 to 0:1, I think..

There's several ways I could implement this, but the two most obvious to
me so far are:

### Method 1

SIMS table:
sim_id

PHONES table:
phone_id
phone_sim_id references sims(sim_id)


### Method 2

PHONES table:
phone_id

SIMS table:
sim_id
sim_phone_id references phones(phone_id)

So problem number 1 is, which way round makes more sense? My first
instinct was method 1. It seemed to make sense to say "the phone has
this sim card", though I suppose you could just as easily turn that on
its head.

Problem number 2 is, using either method above can break the rules and
create an impossible situation. For example, several phones could
reference the same sim card in method 1, and several sim cards could
reference the same phone in method 2. Would the proper way to ensure
integrity in this case be to add a "unique" modifier to the reference
field? e.g:

phone_sim_id references sims(sim_id) unique

Any advice on the above would be much appreciated.

Thanks,

Mark.

Mark S. (UK) wrote:

> Hi all,
> I have 2 tables, "sims" (as in mobile phone sim cards) and "phones" (as
> in mobile phones). I can add any number of sim cards to the "sims"
> table and any number of mobile phones to the "phones" table. Simple enough.
>
> Rule 1: A sim card can either be in a phone, or not in a phone. It
> cannot be in more than one phone.
>
> Rule 2: A phone can either have one sim inserted, or no sim inserted.
>
> So the relationship between the phone and sim is 0:1 to 0:1, I think..
>
> There's several ways I could implement this, but the two most obvious to
> me so far are:
>
> ### Method 1
>
> SIMS table:
> sim_id
>
> PHONES table:
> phone_id
> phone_sim_id references sims(sim_id)
>
>
> ### Method 2
>
> PHONES table:
> phone_id
>
> SIMS table:
> sim_id
> sim_phone_id references phones(phone_id)
>
> So problem number 1 is, which way round makes more sense? My first
> instinct was method 1. It seemed to make sense to say "the phone has
> this sim card", though I suppose you could just as easily turn that on
> its head.

Neither of the above handle both 0 cases.


> Problem number 2 is, using either method above can break the rules and
> create an impossible situation. For example, several phones could
> reference the same sim card in method 1, and several sim cards could
> reference the same phone in method 2. Would the proper way to ensure
> integrity in this case be to add a "unique" modifier to the reference
> field? e.g:
>
> phone_sim_id references sims(sim_id) unique
>
> Any advice on the above would be much appreciated.

If one thinks about the relations above as predicates instead of as
tables, one naturally creates a much better design than either of the above.