foreign key to "some rows" of a second table

From: Chris Withers <chris(at)simplistix(dot)co(dot)uk>
To: PostgreSQL General <pgsql-general(at)postgresql(dot)org>
Subject: foreign key to "some rows" of a second table
Date: 2016-02-22 11:03:56
Message-ID: 56CAEB1C.8030102@simplistix.co.uk
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Hi All,<br>
<br>
So, I have a table that looks like this:<br>
<br>
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CREATE TABLE config (<br>
    region    varchar(10),<br>
    name    varchar(10),<br>
    value    varchar(40)<br>
);<br>
<br>
Another looks like this:<br>
<br>
CREATE TABLE tag (<br>
    host    varchar(10),<br>
    type    varchar(10),<br>
    value    varchar(10)<br>
);<br>
<br>
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'?<br>
<br>
cheers,<br>
<br>
Chris<br>
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