foreign key to "some rows" of a second table

[Chris Withers <chris@xxxxxxxxxxxxxxxx>]

Hi All,

So, I have a table that looks like this:

CREATE TABLE config (
    region    varchar(10),
    name    varchar(10),
    value    varchar(40)
);

Another looks like this:

CREATE TABLE tag (
    host    varchar(10),
    type    varchar(10),
    value    varchar(10)
);

What's the best way to set up a constraint on the 'config' table such that the 'region' column can only contain values that exist in the 'tag' table's value column where the 'type' is 'region'?

cheers,

Chris