Re: What is wrong with this PostgreSQL UPDATE statement??

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com>
Cc: Steve Johnson <stevej456(at)gmail(dot)com>, pgsql-sql(at)postgresql(dot)org
Subject: Re: What is wrong with this PostgreSQL UPDATE statement??
Date: 2008-08-23 02:50:10
Message-ID: 23096.1219459810@sss.pgh.pa.us
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Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com> writes:
> On Fri, 22 Aug 2008, Steve Johnson wrote:
>> update certgroups
>> set termgroupname = tg.termgroupname
>> from certgroups c, termgroup tg
>> where (c.days >= tg.mindays) and (c.days <= tg.maxdays);

> In recent PostgreSQL versions I believe this is properly written:
> update certgroups c
> set termgroupname = tg.termgroupname
> from termgroup tg
> where (c.days >= tg.mindays) and (c.days <= tg.maxdays);

Yeah, in PG's eyes the former is creating a cartesian join between two
versions of certgroups. I think MSSQL interprets the FROM reference as
being the same as the update target, but we don't.

> At least as of SQL2003, I think both of the above use extensions,

Correct, the standard disallows a FROM clause altogether; and I'm not
sure that they weren't right. No matter which way you resolve the above
ambiguity, you've still got the problem that the update behavior is
ill-defined if a given target row joins to more than one set of rows
from the other table(s).

regards, tom lane

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