C# C和SQL-sub从表参数中选择

下面是一段代码片段，用于将表作为参数传递给可在SQLServer2008中使用的查询。我对从@custids id中选择id.custid感到困惑。为什么它使用id.custid和@custids id

private static void datatable_example() 
{

   string [] custids = {"ALFKI", "BONAP", "CACTU", "FRANK"};

   DataTable custid_list = new DataTable();
   custid_list.Columns.Add("custid", typeof(String));

   foreach (string custid in custids) {
      DataRow dr = custid_list.NewRow();
      dr["custid"] = custid;
      custid_list.Rows.Add(dr);
   }

   using(SqlConnection cn = setup_connection())
   {
      using(SqlCommand cmd = cn.CreateCommand()) 
      {

         cmd.CommandText =
           @"SELECT C.CustomerID, C.CompanyName
             FROM   Northwind.dbo.Customers C
             WHERE  C.CustomerID IN (SELECT id.custid FROM @custids id)";
         cmd.CommandType = CommandType.Text;

         cmd.Parameters.Add("@custids", SqlDbType.Structured);
         cmd.Parameters["@custids"].Direction = ParameterDirection.Input;
         cmd.Parameters["@custids"].TypeName = "custid_list_tbltype";
         cmd.Parameters["@custids"].Value = custid_list;

         using (SqlDataAdapter da = new SqlDataAdapter(cmd))
         using (DataSet        ds = new DataSet()) {
            da.Fill(ds);
            PrintDataSet(ds);
         }
      }
   }

---

子查询中的“id”只是一个别名

与在main FROM子句中使用“C”作为别名并使用“C.CustomerID”的方式相同

子查询也可以是

 SELECT custid
 FROM @custids