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To: Bryce Nesbitt <bryce1@obviously.com>
cc: pgsql-sql@postgresql.org
Subject: Re: Syntax question: use of join/using with fully qualified table
	name
In-reply-to: <479D2694.3090202@obviously.com> 
References: <479D2694.3090202@obviously.com>
Comments: In-reply-to Bryce Nesbitt <bryce1@obviously.com>
	message dated "Sun, 27 Jan 2008 16:49:24 -0800"
Date: Sun, 27 Jan 2008 20:45:17 -0500
Message-ID: <16133.1201484717@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Bryce Nesbitt <bryce1@obviously.com> writes:
> I've got a join where a certain column name appears twice:
> select username,last_name from eg_member join eg_membership using
> (member_id) join eg_account using (account_id) join eg_person using
> (person_id);
> ERROR:  common column name "person_id" appears more than once in left table

> My first inclination was to fully quality the table name.  Why would
> this not be acceptable syntax?

Because USING is defined as joining common column names.  It is driven
entirely by the column names, not any "qualification", and it needs to
have well-defined behavior.  The SQL92 spec saith

         <named columns join> ::=
              USING <left paren> <join column list> <right paren>

         <join column list> ::= <column name list>

         <column name list> ::=
              <column name> [ { <comma> <column name> }... ]

         <column name> ::= <identifier>

         ...

            b) If a <named columns join> is specified, then every <column
              name> in the <join column list> shall be the <column name>
              of exactly one column of T1 and the <column name> of exactly
              one column of T2.

			regards, tom lane