var innerJoinQuery =
              from category in categories
              join prod in products on category.ID equals prod.CategoryID
                      select new { ProductName = prod.Name, Category = category.Name };

var innerGroupJoinQuery =
 from category in categories
 join prod in products on category.ID equals prod.CategoryID
 into prodGroup
 select new { CategoryName = category.Name, Products = prodGroup };

var leftOuterJoinQuery =
             from category in categories
             join prod in products on category.ID equals prod.CategoryID
             into prodGroup
                     from item in prodGroup.DefaultIfEmpty(new Product{Name =
             string.Empty, CategoryID = 0})
                     select new { CatName = category.Name, ProdName = item.Name };

var q = (from c in db.Customers
         select c.Country
           ).Union(from e in db.Employees
                   select e.Country
           );

var q1 = from s in db.Student
                        where s.ID < 3
                        select s;
var q2 = from s in db.Student
                        where s.ID < 5
                        select s;

 //去掉重復(fù)的
var q = q1.Union(q2);
var r = q.ToList();//ToList之后，會把數(shù)據(jù)查出來在內(nèi)存中操作

//Concat不會去除重復(fù)項目，相當(dāng)于SQL的Union All；
//不去掉重復(fù)的 相當(dāng)于union all，
var q3 = q1.Concat(q2);
var r3 = q3.ToList()

var q = db.Customers.Union(db.Employees).select(d=>d.Country);