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Re: How do you do a negative join?

From: "Tom Hebbron" <news_user(at)hebbron(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: How do you do a negative join?
Date: 2004-03-22 18:06:16
Message-ID: c3n9v5$1imc$1@news.hub.org (view raw or flat)
Thread:
Lists: pgsql-novice
"Jay Davis" <dj00302003(at)yahoo(dot)com> wrote in message 
news:1d17eeb7(dot)0403201332(dot)58f95e91(at)posting(dot)google(dot)com(dot)(dot)(dot)
> There must be a standard SQL method to query multiple
> tables in the following way.  Lets say we have two
> tables, 'allnames' and 'badnames'. We want to get the
> following result:
>
> "select name from allnames where name-is-not-in-badnames;"
>
> Clearly I'm an SQL novice but I haven't found any examples
> of such a query in my beginning SQL books.
>
> Thanks.

Two ways of doing this:

1. Using the EXCEPT syntax provided in PostgreSQL

SELECT name FROM allnames
EXCEPT
SELECT name FROM badnames;

2. Using a LEFT JOIN and a WHERE NULL constraint (work with MySQL as well)
SELECT
    name
FROM allnames a
LEFT JOIN badnames b ON(a.name = b.name)
WHERE b.name IS NULL;

Hope that helps

-- 
Tom Hebbron
www.hebbron.com



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