LINQ没有为inner和outer joins定义任何关键字，但是我们可以用现有LINQ join语法来实现他们。假设我们有一个部门的序列和课程的序列，那么我们可以执行的内部和外部连接如下：
INNER JOIN
通常来讲，JOIN操作的默认行为是“inner join”当一个课程序列加入一个部门序列的时候，我们只能得到那些有一个或者多个相关课程的部门。
使用JOIN运算符：
==========================================================
var query = from d in db.Departments
join c in db.Courses
ond.DepartmentID equals c.DepartmentID
select new
{
DepartmentName = d.Name,
CourseName = c.Title
};
==========================================================
使用JOIN扩展方法：
==========================================================
var query = db.Departments.Join(db.Courses,
d =>d.DepartmentID,
c =>d.DepartmentID,
(d, c) =>new
{
DepartmentName = d.Name,
CourseName = c.Title
});
==========================================================
OUTER JOIN：
我们可以像用SQL的left outer join一样用GroupJoin运算符。编译器通过在Join语句的下一语句加入关键字来区分GroupJoin和Join运算符。，如果我们想要在给定的部门没有一个课程存在的时候枚举项目内部序列的每一个元素是空值，这种工作就留给SelectMany和DefaultIfEnpty扩展方法去做。
运用GroupJoin运算符:
==========================================================
var query = from d in db.Departments
join c in db.Courses
ond.DepartmetnID equals c.DepartmentID into courseList
select new
{
DepartmentName = d.Name,
Courses = courseList
};
==========================================================
运用GroupJoin扩展方法:
==========================================================
var query = db.Departments.GroupJoin(db.Courses,
d =>d.DepartmentID,
c =>c.DepartmentID,
(d, courseList) =>new
{
DepartmentName = d.Name,
Courses = courseList
});
==========================================================
运用SelectMany和DefaultIfEmpty:
==========================================================
var query = from d in db.Departments
join c in db.Courses
ond.DepartmentID equals c.DepartmentID into courseList
from cl in courseList.DefaultIfEmpty()
select new
{
DepartmentName = d.Name,
CourseName = cl == null ?string.Empty :cl.Title
}
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相关帖子：
http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/fb6a48ea-cd98-4304-95a9-bd4534523fec
http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/b6f2beff-ccdb-419d-9464-08a977d1bce5
http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/541706be-8d21-4618-a0c0-72fa2ffdd047
相关参考：
http://codeexperiment.com/post/LINQ-To-SQL-Outer-Joins.aspx
VB.NET查询：
http://www.codeguru.com/vb/vb_linq/article.php/c15057__1/Implementing-a-Left-Join-with-LINQ.htm