[odb-users] Join Between Tables

[Boris Kolpackov]

Hi Tarik,

Tarik BENZ <tenchu.tarik at hotmail.fr> writes:

> I Have three tables that I mapped, and I would like to know
> how to do a join between them. Do I necessarily need to create a view?

ODB automatically creates a JOIN in two cases (well, technically, there
is a third case: polymorphic objects, but that is not generally visible
to the end user):

1. When there is a relationship between objects (i.e., one points to
   the other). In this case, ODB "JOIN's in" the pointed-to objects in
   queries so that you can refer to data members from these objects.
   For example:

   db.query<employee> (query<employee>::employer->name == "Example, Inc");

   Note that this JOIN'ing of related objects is only done one level
   deep.

2. In views that are based on more than one object (or database
   table). There is a lot of flexibility in views and you can JOIN
   objects and/or database tables using relationships and/or custom
   conditions.

So, unless the JOIN that you are looking for naturally follows from
the relationship between your objects, the views are probably your
best choice.

Boris