Re: FAQ addition: deleteing all but one unique row

You have still not addressed Tom's initial complaint about the delete
deleting all but one row in the table. Shouldn't there be a "WHERE id=12"
on the delete as well?

Kris Jurka

On Wed, 12 Feb 2003 greg(at)turnstep(dot)com wrote:

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> Second shot at a small doc patch, this time with testing. :)
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> Greg Sabino Mullane greg(at)turnstep(dot)com
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> Index: FAQ.html
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> RCS file: /projects/cvsroot/pgsql-server/doc/src/FAQ/FAQ.html,v
> retrieving revision 1.164
> diff -c -r1.164 FAQ.html
> *** FAQ.html 2002/12/05 05:47:44 1.164
> --- FAQ.html 2003/02/10 15:08:00
> ***************
> *** 139,144 ****
> --- 139,145 ----
> temporary tables in PL/PgSQL functions?<BR>
> <A href="#4.27">4.27</A>) What replication options are available?<BR>
> <A href="#4.28">4.28</A>) What encryption options are available?<BR>
> + <A href="#4.29">4.29</A>) How can I delete all but one identical row?<BR>
>
>
> <H2 align="center">Extending PostgreSQL</H2>
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> *** 1381,1386 ****
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> <I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
> <LI>The server can run using an encrypted file system.</LI>
> </UL>
> +
> + <H4><A name="4.29">4.29</A>) How can I delete all but one identical row?<BR>
> + </H4>
> + <P>Sometimes you have rows that are so identical that a simple WHERE clause cannot
> + distinguish them apart. Each row always has a unique system column named
> + <CODE><SMALL>ctid</SMALL></CODE> that can be used to differentiate them. Use
> + <SMALL>LIMIT 1</SMALL> to get back the ctid of one of the identical rows,
> + then remove all matching rows except the one with that particular ctid:</P>
> + <PRE>
> + DELETE FROM mytable WHERE NOT ctid =
> + (SELECT ctid FROM mytable WHERE id=12 LIMIT 1);
> + </PRE>
> +
> + <P>In the above example, all rows in the table named 'mytable' having a value
> + of 12 in the 'id' column will be deleted except for one. Exactly
> + which row is kept should not matter, as they are all otherwise identical.
> + The system column <CODE><SMALL>oid</SMALL></CODE> can also be used, but
> + because tables can be created without an oid column, the use of ctid
> + is preferred.</P>
>
> <HR>
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