Обсуждение: Originally created and last_mod by whom and when ?

Hello,

In a web app (Pg 8.2.4 + php) I have product and other tables with fields like

product_created timestamp without time zone
product_created_user_id integer
product_last_mod timestamp without time zone
product_last_mod_user_id integer

The person who last modified an item can obviously be someone else who 
originally created it.

I can get the names and timestamps with two separate queries but how can I do 
a single query to get the names of both ?

product_id | 1
...
product_created_user_id | 1
product_last_mod_user_id | 2

ID 1, created by X / date and time, last_mod by Y / date and time

And a similar query to only one table, users ?

user_id integer
user_forename text
...
user_created timestamp without time zone
user_created_user_id integer
user_last_mod timestamp without time zone
user_last_mod_user_id integer

ID 4, name Z, created by X / date and time, last_mod by Y / date and time

Join, sub select ? I tried some but only managed to get only one name, errors, 
nothing at all or two rows with inner join + union ...

Lotsa thanks for any help,

Aarni
-- 

Aarni Ruuhimäki wrote:
> Hello,
>
> In a web app (Pg 8.2.4 + php) I have product and other tables with fields like
>
> product_created timestamp without time zone
> product_created_user_id integer
> product_last_mod timestamp without time zone
> product_last_mod_user_id integer
>
> The person who last modified an item can obviously be someone else who
> originally created it.
>
> I can get the names and timestamps with two separate queries but how can I do
> a single query to get the names of both ?

Alias the tables, so you can join to the user-table twice.

SELECT p.*, u_cre.username as created_by, u_mod.username as modified_by
FROM  products p
LEFT JOIN  app_users u_cre ON p.product_created_user_id = u_cre.id
LEFT JOIN  app_users u_mod ON p.product_last_mod_user_id = u.mod.id
;


--   Richard Huxton  Archonet Ltd

On Wednesday 14 November 2007 13:28, Richard Huxton wrote:
> Aarni Ruuhimäki wrote:
> > Hello,
> >
> > In a web app (Pg 8.2.4 + php) I have product and other tables with fields
> > like
> >
> > product_created timestamp without time zone
> > product_created_user_id integer
> > product_last_mod timestamp without time zone
> > product_last_mod_user_id integer
> >
> > The person who last modified an item can obviously be someone else who
> > originally created it.
> >
> > I can get the names and timestamps with two separate queries but how can
> > I do a single query to get the names of both ?
>
> Alias the tables, so you can join to the user-table twice.
>
> SELECT p.*, u_cre.username as created_by, u_mod.username as modified_by
> FROM
>    products p
> LEFT JOIN
>    app_users u_cre ON p.product_created_user_id = u_cre.id
> LEFT JOIN
>    app_users u_mod ON p.product_last_mod_user_id = u.mod.id
> ;

Charming ! Many thanks to you Richard.

Aarni
--