LINQ

 1             //INNER JOIN
 2             var query = (from u in db.UserBase
 3                          from g in db.UserGroup
 4                          where u.ID == g.PID
 5                          select u).ToList();
 6             //INNER JOIN
 7             var query = (from a in db.UserBase
 8                          join b in db.UserGroup on a.ID equals b.PID
 9                          select new Test { id = a.ID, pid = b.PID, name = a.LoginName, Group = b.GroupName }
10                         ).ToList();
11 
12             //INNER JOIN
13             var query = (from a in db.UserBase
14                          join b in db.UserGroup on a.ID equals b.PID
15                          join c in db.WorkGroup on b.GroupName equals c.GroupName
16                          select new Test { id = a.ID, pid = b.PID, name = a.LoginName, Group = b.GroupName }
17                          ).ToList();
18             //CROSS JOIN
19             var query = (from u in db.UserBase
20                          from g in db.UserGroup
21                          select u).ToList();
22 
23             //LEFT OUTER JOIN 
24             var query = (from a in db.UserBase
25                          join b in db.UserGroup on a.ID equals b.PID into order
26                          from o in order.DefaultIfEmpty()
27                          //where o != null
28                          select new Test
29                          {
30                              id = a.ID,
31                              Group = o == null ? "null" : o.GroupName,
32                              name = a.LoginName,
33                              pid = o == null ? 0 : o.PID
34                          }
35                        ).ToList();