Re: Foreign key check only if not null?

hi, fks do just that.

you can create your fk with just one command:
alter table xxx add constraint fk_name foreign key (user_id) references
users (id);

parent table's id field should also be of the same type and also it should
be primary key or at least unique.
you can create your pk with (if you don't already have one):
alter table users add constraint pk_users primary key (id);

also i wouldn't use a varchar(100) as a pk field, i would suggest using some
other datatype maybe a bigint, but that always finally depends on the model,
like if there is a really good reason for using a varchar(100), well, it's
your call.

regards,
eduardo

On Mon, Sep 12, 2011 at 10:48 PM, Phoenix Kiula <phoenix(dot)kiula(at)gmail(dot)com>wrote:

> Hi,
>
> I bet this is a simple solution but I have been racking my brains.
>
> I have a column in my table:
>
> user_id varchar(100) ;
>
> This can be NULL, or it can have a value. If it has a value during
> INSERT or UPDATE, I want to check that the user exists against my
> "Users" table. Otherwise, NULL is ok. (Because the functionality in
> question is open to both unregistered and registered users).
>
> Any idea on how I can implement a FOREIGN KEY constraint? Or do I need
> a pre-insert and pre-update RULE for this?
>
> Thanks!
>
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