Re: Query question

[Stephan Szabo <sszabo(at)megazone(dot)bigpanda(dot)com>]

On Thu, 22 May 2008, Medi Montaseri wrote:

> Hi,
> I can use some help with the following query please.
>
> Given a couple of tables I want to do a JOIN like operation. Except that one
> of the columns might be null.
>
> create table T1 ( id serial, name varchar(20) );
> create table T2 ( id serial, name varchar(20) );
> create table T1_T2 ( id serial, t1_id integer not null , t2_id integer );
>
> Now I'd like to show a list of records from T1_T2 but reference T1 and T2
> for the names instead of IDs. But T1_T2.t2_id might be null
>
> select T1_T2.id, T1.name , T2.name from T1, T2, T1_T2
> where T1_T2.t1_id = T1.id and T1_T2.t2_id = T2.id

What would you want it to do if T1_T2.t2_id has a value that isn't in T2?
And should it do it for both T2 and T1? If using a NULL name is okay for
both, you can look at outer joins, something like:

select T1_T2.id, T1.name, T2.name from
 T1_T2 left outer join T1 on (T1_T2.t1_id = T1.id)
 left outer join T2 on (T1_T2.t2_id = T2.id)

T1_T2 left outer join T1 on (T1_T2.t1_id = T1.id) will for example give
you a row even if there's not a row in T1 with T1.id being the same as
T1_T2.t1_id.  In that case, you'll get the fields from T1_T2 and NULLs for
the fields from T1. The same between that table and T2 occurs with the
second outer join.