Received: from localhost (postgresql.org [64.49.215.8]) by postgresql.org (Postfix) with ESMTP id 78046476138 for ; Thu, 12 Dec 2002 02:13:30 -0500 (EST) Received: from sss.pgh.pa.us (unknown [192.204.191.242]) by postgresql.org (Postfix) with ESMTP id C9FDB475F3D for ; Thu, 12 Dec 2002 02:13:29 -0500 (EST) Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) by sss.pgh.pa.us (8.12.6/8.12.6) with ESMTP id gBC7DTpd004082; Thu, 12 Dec 2002 02:13:29 -0500 (EST) To: "A.M." Cc: pgsql-sql@postgresql.org Subject: Re: Getting the latest unique items In-reply-to: <9352B184-0D90-11D7-BA9C-0030657192DA@cmu.edu> References: <9352B184-0D90-11D7-BA9C-0030657192DA@cmu.edu> Comments: In-reply-to "A.M." message dated "Thu, 12 Dec 2002 00:14:22 -0500" Date: Thu, 12 Dec 2002 02:13:29 -0500 Message-ID: <4081.1039677209@sss.pgh.pa.us> From: Tom Lane X-Virus-Scanned: by AMaViS new-20020517 X-Archive-Number: 200212/144 X-Sequence-Number: 10732 "A.M." writes: > When I try to run the following query: > select distinct on(student_gradedmaterial.id) ... ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > ... order by submittime desc; ^^^^^^^^^^^^^^^^^^^ > I get the following error: > ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY > expressions Indeed. You might benefit from reading the DISTINCT ON usage example given in the SELECT reference page, http://www.ca.postgresql.org/users-lounge/docs/7.3/postgres/sql-select.html (see under "Description"). Basically, DISTINCT ON is *only* useful when designed hand-in-hand with an ORDER BY ... regards, tom lane