Linq实现LEFT OUT JOIN

Linq实现LEFT OUT JOIN

原文地址：http://geekswithblogs.net/AzamSharp/archive/2008/04/07/121103.aspx

Suppose you have a tblRoom and tblUserInfo. Now, you need to select all the rooms regardless of whether the room has user information or not. This calls for a LEFT JOIN which will select everything from the LEFT side (the room side) regardless of the join on the right side. Here is the example.

假如你有两张表tblRoom（房间表）和tblUserInfo（住户表）。现在你需要检索出所有房间的信息，而不管这个房间是否有人居住。这就需要进行LEFT JOIN（左外连接），左外连接会检索出LEFT JOIN左边表中的所有行，而不管右边的表是否有匹配项。下面是一个例子：

var list = from r in dc.tblRooms
                          join ui in dc.tblUserInfos
                          on r.UserName equals ui.UserNameinto userrooms

                          from ur in userrooms.DefaultIfEmpty()

                          select new
                           {
                               FirstName = (ur.FirstName == null) ? "N/A" : ur.FirstName,
                               LastName = (ur.LastName == null) ? "N/A" : ur.LastName,
                               RoomName = r.Name
                           };

The anonymous type replaces the "null" FirstName and LastName with "N/A" (not available).

使用"N/A"（不可得）代替 FirstName 和 LastName 值为"null"的情况。

 

另附：Linq实现多个表 LEFT JOIN 如下

来自：http://hi.baidu.com/xuejianxiyang/item/0c9df0175c8dbdfaddeecae3

目标SQL语句（多表 LEFT JOIN 查询）

目标SQL语句（多表 LEFT JOIN 查询）SELECT id, name, jname, cname     FROM userinfo u     LEFT JOIN job j on u.job = j.jid     LEFT JOIN city c on u.city = c.cid

Linq To Sql 实现三个表 LEFT JOIN 如下：

var list = (from u in dc.userinfos    join j in dc.jobs on u.job equals j.jid into j_joinfrom x in j_join.DefaultIfEmpty()    join c in dc.cities on u.city equals c.cid into c_joinfrom v in c_join.DefaultIfEmpty()select new{    id = u.id,    name = u.name,    jname = x.jname,    cname = v.cname,    /*u1=u,x1=x,v1=v*/    //不要用对象的方式 因为对象可能为null那么对象.属性就会抛异常}).ToList();for (var i = 0; i < list.Count(); i++){    Console.WriteLine(list[i].name + '\t' + list[i].jname + '\t' + list[i].cname); //字段为null不报异常    //Console.WriteLine(list[i].u1.name+'\t'+list[i].x1.jname+'\t'+list[i].v1.cname+"\r\n"); //对象x1 v1 有可能为null 抛异常}Console.ReadLine();

我用Linq自己写的3个表 LEFT JOIN 例子：

Emp（员工表）、Dept（部门表）、KqEmp（人员考勤信息表）