Thanks Stephan, 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.
Thanks again Medi On Thu, May 22, 2008 at 2:50 PM, Stephan Szabo <[EMAIL PROTECTED]> wrote: > 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. > >
