On Thu, 24 Sep 2009 16:15:07 -0400, justin wrote about Re: [SQL] simple (?) join:
> > >David W Noon wrote:On Thu, 24 Sep 2009 16:16:36 +0100, Gary Stainburn >wrote about [SQL] simple (?) join: > > create table orders ( >o_id serial primary key >... >); > >create table orders_log ( >ol_id serial primary key, >o_id int4 not null references orders(o_id), >ol_timestamp timestamp, >ol_user, >); > >How can I select all from orders and the last (latest) entry from the >orders_log? > >SELECT * FROM orders >WHERE o_id IN (SELECT o_id FROM orders_log > WHERE ol_timestamp = (SELECT MAX(ol_timestamp) FROM orders_log)); > >No joins required. > >I don't think that is what he is requesting. I read it he also wants >the timestamp included in the result set That shouldn't be too much of a problem. >A nested query > >Select > orders.*, > (SELECT MAX(ol_timestamp) FROM orders_log where orders_log.o_id = > orders.oid) >From orders That won't give the desired results. I don't think the SQL parser will even accept it. >Still another option is using a join > >Select > orders.*, ol_timestamp > From orders > left join (SELECT MAX(ol_timestamp), o_id FROM orders_log group by > o_id) as JoinQuery on JoinQuery.o_id = orders.o_id That won't give the desired results either. If you change "left" to "inner" you will be closer though. Both of your queries will retrieve the entire orders table with a timestamp of some sort from the orders_log table. >The second one should be faster Try this for an elegant approach: WITH maxi AS (SELECT MAX(ol_timestamp) AS ts FROM orders_log) SELECT o.*, maxi.ts FROM orders AS o WHERE o.o_id IN (SELECT o_id FROM orders_log WHERE ol_timestamp = maxi.ts); This caches the latest timestamp in a temporary table so that it can both appear in the result set and be used to constrain the selection from the orders_log table. -- Regards, Dave [RLU #314465] ======================================================================= david.w.n...@ntlworld.com (David W Noon) ======================================================================= -- Sent via pgsql-sql mailing list (pgsql-sql@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-sql
