My real DDL include a forign key reference to T2.id and since I am ok with
NULL value then the "left outer join" indeed have solved the problem.
On Thu, May 22, 2008 at 2:50 PM, 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
> > 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.
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