The first one doesn't return anything, but the last one (ignoring the name space) it's OK. I'll use that.

Thank you very much

you're right, it's not very clear...it could be a dynamic query that does a join only for a value of id_obj.

I don't know how to write a dynamic query ,for...

no, because the primary key is id_obj , category

I could have the same id_obj with 2 different categories. So, for the category 1 I have to do  join with a...

CREATE  TABLE #An_Obj
(id_obj nvarchar(5),
id_category int)

CREATE  TABLE #JoinTable
(
id_category int,
dbjoin nvarchar(50),
schemajoin nvarchar(50),
tablejoin nvarchar(50),
fieldjoin nvarchar(50)
)

insert into #An_Obj values ('a1',1)
insert into #An_Obj values ('b2',2)

insert into #JoinTable values (1,'mydb','dbo','db_a','id_a')
insert into...