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Re: JOIN question with multiple records

From: Richard Huxton <dev(at)archonet(dot)com>
To: "Scott, Casey" <Casey(dot)Scott(at)wizards(dot)com>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: JOIN question with multiple records
Date: 2006-01-05 08:40:56
Message-ID: 43BCDB98.7030005@archonet.com (view raw or flat)
Thread:
Lists: pgsql-sql
Scott, Casey wrote:
> I have 2 tables. One containing information about servers, and the other
> containing information about IP addresses.
> 
> E.G.
> Server table:
> 
> 
> name				mac
> mac2
> -------------------------------------------------------------
> SERVER1		00:0d:56:ba:ad:92
> SERVER2		00:0d:56:ba:ad:93
> 00:0d:56:ba:ad:96
> SERVER3		00:0d:56:ba:ad:94
> SERVER4		00:0d:56:ba:ad:95
> 00:0d:56:ba:ad:97


I think you've got the design of this table wrong.

It looks like you're leaving mac2 NULL where the server has only one 
network-card. This is wrong - mac2 is not "unknown" it is "card not 
present" or similar (and the type of the column should then be not 
mac-address but mac-address-and-not-present).

I'm also not sure how you will handle the case when a server has 3 
network-cards. Also, if you want to know which server has a specific 
mac-addr then you'll need to check two columns with your current design.

If possible I'd suggest reworking the table to something like: (name, 
card-id, mac-addr) and you'd then have:
SERVER2  0  00:0d:56:ba:ad:93
SERVER2  1  00:0d:56:ba:ad:96
...

Then a crosstab function / case statement can reformat your query output 
as required.

> SELECT servers.name,addresses.ipaddr,servers.application_mgr FROM
> servers LEFT JOIN addresses ON addresses.mac = servers.mac OR
> addresses.mac = servers.mac2

Well, if you can't change the structure of your tables you could do 
something like:

SELECT ...
FROM  servers s
LEFT JOIN  addresses a1
ON  s.mac = a1.mac
LEFT JOIN addresses a2
ON s.mac = a2.mac

The crucial bit is aliasing the "addresses" table twice.
-- 
   Richard Huxton
   Archonet Ltd

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