Question

我有这个简单的SQL查询：

SELECT  CustomerName, OrderId 
FROM    Customer c
        LEFT JOIN Orders o ON o.CustomerId = c.CustomerId

我需要从查询结果中填充我的Customer实体：

public class Customer
 {
    public string Name { get; set; }
    public IEnumerable<int> OrderIds { get; set; }
 }

我正在使用SqlDataReader读取数据：

using(SqlDataReader rdr = cmd.ExecuteReader())
{      
    while (rdr.Read())
    {
     Customer c = new Customer();
     c.Name = rdr["CustomerName"].ToString();      
     yield return c;
    }
}

问题：在不使用ORM的情况下，填写 OrderIds 属性的最简单，最简洁的方法是什么？左连接导致查询返回每个客户多行（因为每个客户有多个订单）所以现在逐行读取它就像上面的代码一样不起作用？有没有办法填充该实体，仍然使用收益率返回延期执行？

Answer 1

第一版：

var customersRDR = from rCustomers in rdr.Cast<DbDataRecord>()
                   group rCustomers by rCustomers["CustomerName"] into custGroups
                   select new Customer
                   {
                      Name = (string)custGroups.Key,
                      OrderIds = from c in custGroups select (Int32)c["OrderId"]
                   };

第二版：

DataTable dt = new DataTable();
dt.Load(rdr);

var customers = from c in dt.AsEnumerable()
                group c by c["CustomerName"] into custGroups
                select new Customer{
                   Name = custGroups.Key.ToString(),
                   OrderIds = from c in custGroups select Convert.ToInt32(c["OrderId"])
               };

第3版：

DataTable dt = new DataTable();
dt.Load(rdr);

var customerGroups = dt.AsEnumerable()
                     .GroupBy(c => c["CustomerName"]);

foreach (var customer in customerGroups){
     Customer cust = new Customer();
     cust.Name = customerGroup.Key.ToString();
     cust.OrderIds = from c in customerGroup select Convert.ToInt32(c["OrderId"]);
    }

Answer 2

List<int> orderIds = (from IDataRecord r in rdr
                      where (string) r["CustomerName"] == c.Name
                      select (int)r["OrderIds"]).ToList();

修改

如果没有ORM，这有点棘手。然而，另一种选择是使用多个阅读器。它需要您有两个查询 - 一个用于客户，一个用于订单。举个例子：

private void GetCustomerWithOrders() { using (var conn = new SqlConnection("your connection string")) { using (var cmd = conn.CreateCommand()) { conn.Open(); cmd.CommandText = "SELECT CustomerName, CustomerId FROM Customer"; using (var reader = cmd.ExecuteReader()) { while (reader.Read()) { var customerId = reader.GetInt32(1); // CustomerId List<int> orders = GetOrders(customerId); } } } } } private List<int> GetOrders(int customerId) { var orders = new List<int>(); using (var conn = new SqlConnection("your connectionstring")) { using (var cmd = new SqlCommand("SELECT OrderId FROM Orders WHERE CustomerId = @CustomerId", conn)) { var param = new SqlParameter { ParameterName = "@CustomerId", Value = customerId }; cmd.Parameters.Add(param); using (var reader = cmd.ExecuteReader()) { while (reader.Read()) { var orderId = reader.GetInt32(0); // OrderId orders.Add(orderId); } } } } return orders; }

Answer 3

var dataReader = cmd.ExecuteReader();

var dataTable = new DataTable();

dataTable.Load(dataReader);

如何从SqlDataReader填充集合属性？

3 个答案: