Question

DDL for Database Tables:

    Users:     id - int - identity     name - varchar - unique      PCs:     id - int - idnetity     name - varchar - unique     userid - FK to Users      Apps:     id - int - identity     name - varchar     pcid - FK to PCs 

I created a DataContext using the Linq To SQL designer in Visual Studio 2008.

I want to perform this query:

select   users.name,   pcs.name,   apps.name from   users u   join pcs p on p.userid = u.id   join apps a on a.pcid = p.id 

I was told in another thread where I posted an answer that the following was incorrect and that it created a cross-join.

var query = from u in db.Users // gets all users         from p in u.PCs // gets all pcs for user         from a in p.Apps // gets all apps for pc         select new         {             username = u.Name,             pcname = p.Name,             appname = a.Name         }; 

When I execute this query I get the correct results. A cross-join with two records in each table should return 8 records but my query correctly returns the two records.

Am I lucky, or is the person telling me that I’m wrong confused?

Answer 1

That will work. It’s normal because the second ‘from’ inquires in the PC’s set of the users and the third ‘from’, the Apps set of the pcs. And I guess that is not going generate a cross join as T-Sql, because the conditions are already defined in the foreign keys and associations.

But I guess this should be the syntactically you want by inner join;

var query = from u in db.Users             join p in db.PCs on p.UserId == u.Id             join a in db.Apps on a.PCId == p.Id             select new             {                 username = u.Name,                 pcname = p.Name,                 appname = a.Name             };