使用 lambda 表达式左连接多个表

Question

我有 2 张桌子：

规范 {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
}

请协助。

数据就像这样说：

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，为了方便起见，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();

扩展GroupJoin方法可帮助您获取包含起始表的所有元素以及连接表的元素列表的元组。

现在，如果您枚举结果：

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