使用 LINQ 将多行连接成单行（CSV 属性）

Question

我正在寻找与 Sybase 的 LIST() 或 MySQL 的 group_concat() 等效的 LINQ

它会转换：

User  Hobby
--------------
Bob   Football 
Bob   Golf 
Bob   Tennis 
Sue   Sleeping 
Sue   Drinking

收件人：

User  Hobby
--------------
Bob   Football, Golf, Tennis 
Sue   Sleeping, Drinking

Answer 1

所有答案都不够好；

因为这是一个数据库查询，但我们所有人都只是在内存中这样做；

区别是内存中的某些操作会发生错误can't trans to store expression;

var list = db.Users.GroupBy(s=>s.User).
             select(g=>new{user=g.Key,hobbys=g.select(s=>s.Hobby)}); // you can just do that from db

var result=list.ToList(); // this is important,to query data to memory;

var result2 = result.select(g=>new{user=g.Key,hobbyes=string.join(",",g.hobbyes)}; //then,do what you love in memory

Answer 2

否则我们可以执行以下操作-

var users = new[]
                {
                new { User="Bob", Hobby="Football" },
                new { User="Bob", Hobby="Golf" },
                new { User="Bob", Hobby="Tennis" },
                new { User="Sue", Hobby="Sleeping" },
                new { User="Sue", Hobby="Drinking" },
                };

                var userList = users.ToList();
                var ug = (from user in users
                          group user by user.User into groupedUserList
                          select new { user = groupedUserList.Key, hobby = groupedUserList.Select(g =>g.Hobby)});

                var ug2 = (from groupeduser in ug
                          select new{ groupeduser.user, hobby =string.Join(",", groupeduser.hobby)});

Answer 3

在一个 Linq 语句中完成。我没有办法推荐代码，但它表明它可以完成。

            var groupedUsers = from user in users
                           group user by user.User into userGroup
                           select new
                           {
                               User = userGroup.Key,
                               userHobies =
                                   userGroup.Aggregate((a, b) => 
                                       new { User = a.User, Hobby = (a.Hobby + ", " + b.Hobby) }).Hobby
                           }
                            ;
        foreach (var x in groupedUsers)
        {
            Debug.WriteLine(String.Format("{0} {1}", x.User, x.userHobies));
        }

Answer 4

关于您问题的_concat 方面，使用：

static class EnumerableExtensions 
{  
    public static String AsJoined( this IEnumerable<String> enumerable )
    {
        return AsJoined( enumerable, "," );
    }

    public static String AsJoined( this IEnumerable<String> enumerable, String separator )
    {
        return String.Join( separator, enumerable.ToArray() );
    }
}

bruno conde 和 Jon Skeet 的答案中的输出 foreach 可以变成：

Console.WriteLine( "User:\tHobbies");
foreach ( var group in groupedUsers )
    Console.WriteLine( "{0}:\t{1}", group.Key, group.Select( g => g.Hobby ).AsJoined( ", " ) );

...你会得到你要求的精确结果输出格式（是的，我知道其他人已经解决了你的问题，但很难抗拒！）

Answer 5

看看这个解决方案是否对你有帮助：

List<User> users = new List<User>() 
{ 
    new User {Name = "Bob", Hobby = "Football" },
    new User {Name = "Bob", Hobby = "Golf"},
    new User {Name = "Bob", Hobby = "Tennis"},
    new User {Name = "Sue", Hobby = "Sleeping"},
    new User {Name = "Sue", Hobby = "Drinking"}
};

var groupedUsers = from u in users
         group u by u.Name into g
         select new
         {
             Name = g.First<User>().Name,
             Hobby = g.Select(u => u.Hobby)
         };


foreach (var user in groupedUsers)
{
    Console.WriteLine("Name: {0}", user.Name);
    foreach (var hobby in user.Hobby)
    {
        Console.WriteLine("Hobby: {0}", hobby);
    }
}

Answer 6

这就是 GroupBy 运算符。你在使用 LINQ to Objects 吗？

这是一个例子：

using System;
using System.Collections.Generic;
using System.Linq;

public class Test
{
    static void Main()
    {
        var users = new[]
        {
            new { User="Bob", Hobby="Football" },
            new { User="Bob", Hobby="Golf" },
            new { User="Bob", Hobby="Tennis" },
            new { User="Sue", Hobby="Sleeping" },
            new { User="Sue", Hobby="Drinking" },
        };

        var groupedUsers = users.GroupBy(user => user.User);

        foreach (var group in groupedUsers)
        {
            Console.WriteLine("{0}: ", group.Key);
            foreach (var entry in group)
            {
                Console.WriteLine("  {0}", entry.Hobby);
            }
        }
    }
}

这样分组 - 你能自己管理其余的吗？