使用 lambda 表达式左连接多个表

Question

我有 2 张桌子：

specs {specId, desc, createdby, lastupdatedby}
用户 {userid, 用户名}

我希望下面的 linq 查询需要用纯 lambda 表达式编写

    from spec in specs
    from user in users.where(x => x.userId== spec.createdby).DefaultIfEmpty()
    from updatedUser in users.where(x => x.userId== spec.lastupdatedbyby).DefaultIfEmpty()
select new {
spec = spec
user = user,
updatedUser = updatedUser
}

请帮忙。

数据就像说：

spec[{1, test, 1234, 2345},{2, test1, 1234, null}]

users[{1234, Allen},{2345, Dwayne}]

所以结果应该是

[{1, test, Allen, Dwayne}, {2, test1, Allen, null}]

Answer 1

让我们从这些类开始：

class Specs {
    public int specId { get; set; }
    public string desc { get; set; }
    public int createdby { get; set; }
    public int lastupdatedby { get; set; }
}

class Users {
    public int userId { get; set; }
    public string username { get; set; }
}

class UpdatedUser {
    public int userId {get; set;}
    public string username { get; set; }
}

现在是 Linq 查询，为方便起见，我创建了一些示例数据：

var specs = new Specs[]
{
    new Specs{specId = 1, desc = "Spec1", createdby=1, lastupdatedby=1},
    new Specs{specId = 2, desc = "Spec2", createdby=2, lastupdatedby=3},    
    new Specs{specId = 3, desc = "Spec3", createdby=3, lastupdatedby=1},
    new Specs{specId = 4, desc = "Spec4", createdby=3, lastupdatedby=3},
};

var user = new Users[]
{
    new Users{userId = 1, username = "User1"},
    new Users{userId = 2, username = "User2"},
};

var updatedUser = new UpdatedUser[]
{
    new UpdatedUser{userId = 1, username = "UpdatedUser1"},
    new UpdatedUser{userId = 2, username = "UpdatedUser2"},         
};

var result = specs
    .GroupJoin(user, 
        s => s.createdby,
        u => u.userId,
    (s, u) => u.Select(x => new {spec = s, user = x})
            .DefaultIfEmpty(new {spec = s, user = (Users)null}))
.SelectMany(g => g)
.GroupJoin(updatedUser,
        firstJoin => firstJoin.spec.lastupdatedby,
        uu => uu.userId,
        (firstJoin, uu) => 
        uu.Select(y => new {spec = firstJoin.spec, user = firstJoin.user, updatedUser = y})
.DefaultIfEmpty(new {spec = firstJoin.spec, user = firstJoin.user, updatedUser = (UpdatedUser) null}))
    .SelectMany(g1 => g1)
    .ToList();

GroupJoin 扩展方法可帮助您获得一个元组，其中包含起始表的所有元素以及连接表的元素列表。

现在如果你列举结果：

result.ForEach(item => {
    Console.WriteLine(item.spec.desc);
    Console.WriteLine(item.user != null ? item.user.username : "NULL");
    Console.WriteLine(item.updatedUser != null ? item.updatedUser.username : "NULL");
    Console.WriteLine();
});

你得到这个：

Spec1
User1
UpdatedUser1

Spec2
User2
NULL

Spec3
NULL
UpdatedUser1

Spec4
NULL
NULL

Answer 2

你可以试试

var list = specs.Join(users,
                s => s.lastupdatedby,
                u => u.userid,
                (s, u) => new { specs = s, users = u })
                .Select(x => new {
                    specId = x.specs.specId,
                    desc = x.specs.desc,
                    createdby=x.specs.createdby,
                    username=x.users.username

                }).ToString();

Answer 3

LEFT JOIN MULTIPLE TABLES
------------------------

create table Rama(rid int,name varchar(80),zip int);
create table Kris(kid int,rid int, Title varchar(80),zip int);

insert into Rama values(1,'Robert Rama' ,10001),
                       (2,'Roger Raju'  ,10002),
                       (3,'Kevin Kali'  ,10003),
                       (4,'Mark Mutu'   ,10004)

insert into Kris values(0,0,'NP' ,10001),
                       (1,1,'VP' ,10001),
                       (2,2,'AVP',10002)

//Lambda expression
//Download https://www.linqpad.net/Download.aspx
//Create tables as given below and connect linqpad to your db
//select C#  statement(s) on linqpad and  run below
var v =
from r in Ramas
join k in Kris
  on new { r.Rid, r.Zip } equals new { k.Rid, k.Zip }
  into resultGroups
from k in resultGroups.DefaultIfEmpty()
select new { r.Rid, r.Name, k.Title };

v.Dump();