Question

有人能举例说明如何使用LINQ / lambda表达式执行左连接操作吗？

Answer 1

MSDN上的LINQ to SQL samples page给出了如何实现此目的的示例。对于LINQ to Objects，代码应该完全相同。

此处的关键是对DefaultIfEmpty的调用。

Dim q = From e In db.Employees _
        Group Join o In db.Orders On e Equals o.Employee Into ords = Group _
        From o In ords.DefaultIfEmpty _
        Select New With {e.FirstName, e.LastName, .Order = o}

如果您需要帮助将其转换为C＃，请询问。

Answer 2

这是LINQ中的example of a left join。

Answer 3

例如：

IQueryable<aspnet_UsersInRole> q = db.aspnet_Roles
                    .Select(p => p.aspnet_UsersInRoles
                        .SingleOrDefault(x => x.UserId == iduser));

将为您提供asp.net成员资格中的角色列表，其中null与指定用户不匹配（iduser密钥）

Answer 4

我发现自己喜欢的方法是将OuterCollection.SelectMany()与InnerCollection.DefaultIfEmpty()结合起来。您可以使用“C＃语句”模式在LINQPad中运行以下内容。

var teams = new[] 
    { 
        new { Id = 1, Name = "Tigers" }, 
        new { Id = 2, Name = "Sharks" }, 
        new { Id = 3, Name = "Rangers" },
    };

var players = new[] 
    { 
        new { Name = "Abe", TeamId = 2}, 
        new { Name = "Beth", TeamId = 4}, 
        new { Name = "Chaz", TeamId = 1}, 
        new { Name = "Dee", TeamId = 2}, 
    };

// SelectMany generally aggregates a collection based upon a selector: from the outer item to
//  a collection of the inner item.  Adding .DefaultIfEmpty ensures that every outer item
//  will map to something, even null.  This circumstance makes the query a left outer join.
// Here we use a form of SelectMany with a second selector parameter that performs an
//  an additional transformation from the (outer,inner) pair to an arbitrary value (an
//  an anonymous type in this case.)
var teamAndPlayer = teams.SelectMany(
    team => 
        players
        .Where(player => player.TeamId == team.Id)
        .DefaultIfEmpty(),
    (team, player) => new 
        { 
             Team = team.Name, 
             Player = player != null ? player.Name : null 
        });

teamAndPlayer.Dump();

// teamAndPlayer is:
//     { 
//         {"Tigers", "Chaz"},
//         {"Sharks", "Abe"},
//         {"Sharks", "Dee"},
//         {"Rangers", null}
//     }

在尝试这个时，我发现有时你可以在匿名类型的实例化中省略player的空值检查。我认为在数据库上使用LINQ-to-SQL时就是这种情况（而不是这里的这些数组，我认为这使得它成为LINQ到对象之类的东西。）我认为在LINQ中省略了null检查-to-SQL因为查询被转换为SQL LEFT OUTER JOIN，它直接跳过与外部项连接null。（请注意，匿名对象的属性值必须是可空的;因此，如果您想安全地包含int，那么您需要：new { TeamId = (int?)player.TeamId }。

Answer 5

我试图重现着名的左连接，其中b键为null，我得到的结果就是这种扩展方法（只需要一点点想象力就可以将它修改为只做左连接）：

    public static class extends
{
    public static IEnumerable<T> LefJoinBNull<T, TKey>(this IEnumerable<T> source, IEnumerable<T> Target, Func<T, TKey> key)
    {
        if (source == null)
            throw new ArgumentException("source is null");

        return from s in source
               join j in Target on key.Invoke(s) equals key.Invoke(j) into gg
               from i in gg.DefaultIfEmpty()
               where i == null
               select s;
    }
}

使用LINQ左连接

5 个答案: