--构造测试数据： 只作演示用
CREATE TABLE [dbo].[Tim_LinqTable](
[Id] int PRIMARY KEY IDENTITY(1,1) NOT NULL,
[Name] [varchar](50) NOT NULL,
[Parent] int NOT NULL,
)
GO
 
INSERT INTO [Tim_LinqTable]  
SELECT ‘A‘,0 UNION ALL
SELECT ‘A1‘,1 UNION ALL
SELECT ‘A2‘,1 UNION ALL
SELECT ‘B1‘,2 UNION ALL
SELECT ‘B2‘,3 UNION ALL
SELECT ‘C1‘,4 UNION ALL
SELECT ‘C2‘,4 UNION ALL
SELECT ‘D1‘,5 UNION ALL
SELECT ‘D2‘,5 UNION ALL
SELECT ‘D3‘,5 
GO
 
WITH temp
AS
(
SELECT * FROM [Tim_LinqTable]  WHERE Parent = 3
UNION ALL
SELECT m.* FROM [Tim_LinqTable]  AS m
INNER JOIN temp AS child ON m.Parent = child.Id
)
SELECT * FROM temp
GO
 
--查询 Parent=3 的所有子数据结果如下:
Id          Name                                               Parent
----------- -------------------------------------------------- -----------
5           B2                                                 3
8           D1                                                 5
9           D2                                                 5
10          D3                                                 5
 
(4 row(s) affected)

//好，下边来看看用C#怎么实现上边的SQL语句吧：
void Main()
{
     var query=GetClassID(3);
     Console.WriteLine("Id\tName\tParent");
     query.ToList().ForEach(q=>Console.WriteLine("{0}\t{1}\t{2}",q.Id,q.Name,q.Parent));
     /*
        Id       Name       Parent
        5        B2         3
        8        D1         5
        9        D2         5
       10        D3         5
     */
}
public IEnumerable<Tim_LinqTable> GetClassID(int p_id)
{
            var query = from c in this.Tim_LinqTables
                        where c.Parent  == p_id
                        select c;
         
           return  query.ToList().Concat(query.ToList().SelectMany(t => GetClassID(t.Id)));              
}