linq 查询（sql 到 linq）

Question

拥有三个表（Item、Purchase、PurchaseItem），我正在尝试编写一个 linq 查询来查找一个项目包含多少有效购买。

在标准 SQL 中，我会使用带有计数的子查询，但我似乎无法获得 linq 查询来执行此操作。

SQL：

select i.name, 
InTrade = (select count(*) from Purchase p join PurchaseItem pi on p.id = pi.PurchaseId where p.Isvalid = 1 and pi.ItemId = i.Id)
from Item i

林克：

from i in Items select new 
{
    i.Name,
    InTrade = from pi in PurchaseItems where pi.ItemId == i.ID
        select new 
            {
                pi,
                Purchase = from p in Purchases where p.ID == pi.PurchaseId
                select new { p }
            }
}

这里我试图在一个查询中检索我的所有数据，我可能需要另一个查询才能真正得到我想要的。使用这种方法，我可能会走上一条麻烦的道路，所以也许有人可以阐明在 linq 中实现这一目标的“最佳实践”。

我为这个例子准备了一些测试数据：

if exists (select * from sysobjects where name = 'PurchaseItem') drop table PurchaseItem
if exists (select * from sysobjects where name = 'Item') drop table Item
if exists (select * from sysobjects where name = 'Purchase') drop table Purchase
go

create table Item
(
    ID int not null primary key,
    Name nvarchar(30) not null
)

create table Purchase
(
    ID int not null primary key,    
    Name nvarchar(30) not null,
    ConfirmedDate datetime,
    Isvalid bit not null
)

create table PurchaseItem
(
    ID int not null primary key,
    PurchaseId int null references Purchase(ID),
    ItemId int null references Item(ID)
)

insert Item values (1, 'Bread')
insert Item values (2, 'Beer')
insert Item values (3, 'Wine')
insert Item values (4, 'Milk')
insert Item values (5, 'Apple')
insert Item values (6, 'Steak')
insert Item values (7, 'Hamburger')
insert Item values (8, 'Vinegar')

insert Purchase values (1, 'Purchase 1', null, 1)
insert Purchase values (2, 'Purchase 2', null, 0)
insert Purchase values (3, 'Purchase 3', '2008-1-7', 1)

insert PurchaseItem values (1, 1, 1)
insert PurchaseItem values (2, 1, 2)
insert PurchaseItem values (3, 1, 3)
insert PurchaseItem values (4, 1, 4)
insert PurchaseItem values (5, 1, 5)
insert PurchaseItem values (6, 1, 6)
insert PurchaseItem values (7, 2, 2)
insert PurchaseItem values (8, 3, 2)
insert PurchaseItem values (9, 3, 1)
insert PurchaseItem values (10, 3, 5)

Answer 1

我认为您可以通过几种方法来完成此操作，只需对数据库进行一次查询即可。以下是我能想到的两个：

使用商品与其购买之间的双向关联：

var resultUsingSubItems = context.PurchaseItems.Select(x => new
    {
        Item = x,
        ValidPurchases = x.Purchases.Count(y => y.IsValid)
    });

还有另一种方法，按项目分组：

var resultUsingGrouping = context.Purchases.GroupBy(x => x.Item).Select(
        g => new
            {
                Item = g.Key,
                ValidPurchases = g.Count(y => y.IsValid)
            });

请使用SQL profiler 确保 Linq2SQL 将其转换为单个 SQL 查询。

Answer 2

您也可以这样做：

var results = from i in db.Items
              select new InTradeResult
              {
                ProductName = i.Name,
                InTradeCount = i.PurchaseItems.Count(pi => pi.Purchase.IsValid)
              };

这是一个显示结果的示例控制台应用程序：

class Program
{
  static void Main(string[] args)
  {
    using (var db = new Context())
    {
      var results = from i in db.Items
                    select new InTradeResult
                    {
                      ProductName = i.Name,
                      InTradeCount = i.PurchaseItems.Count(pi => pi.Purchase.IsValid)
                    };

      foreach (var result in results)
      {
        System.Console.WriteLine("{0} - {1}", result.InTrade, result.ProductName);
      }

      System.Console.ReadKey();
    }
  }
}

public class InTradeResult
{
  public string ProductName { get; set; }
  public int InTradeCount { get; set; }
}

public class Context : IDisposable
{
  public IList<Item> Items { get; set; }
  public IList<Purchase> Purchases { get; set; }
  public IList<PurchaseItem> PurchaseItems { get; set; }

  public Context()
  {
    Items = new List<Item>();
    Items.Add(new Item { ID = 1, Name = "Bread" });
    Items.Add(new Item { ID = 2, Name = "Beer" });
    Items.Add(new Item { ID = 3, Name = "Wine" });
    Items.Add(new Item { ID = 4, Name = "Milk" });
    Items.Add(new Item { ID = 5, Name = "Apple" });
    Items.Add(new Item { ID = 6, Name = "Steak" });
    Items.Add(new Item { ID = 7, Name = "Hamburger" });
    Items.Add(new Item { ID = 8, Name = "Vinegar" });

    Purchases = new List<Purchase>();
    Purchases.Add(new Purchase { ID = 1, Name = "Purchase 1", ConfirmedDate = null, IsValid = true });
    Purchases.Add(new Purchase { ID = 2, Name = "Purchase 2", ConfirmedDate = null, IsValid = false });
    Purchases.Add(new Purchase { ID = 3, Name = "Purchase 3", ConfirmedDate = null, IsValid = true });

    PurchaseItems = new List<PurchaseItem>();
    PurchaseItems.Add(new PurchaseItem(1, Purchases[0], Items[0]));
    PurchaseItems.Add(new PurchaseItem(2, Purchases[0], Items[1]));
    PurchaseItems.Add(new PurchaseItem(3, Purchases[0], Items[2]));
    PurchaseItems.Add(new PurchaseItem(4, Purchases[0], Items[3]));
    PurchaseItems.Add(new PurchaseItem(5, Purchases[0], Items[4]));
    PurchaseItems.Add(new PurchaseItem(6, Purchases[0], Items[5]));
    PurchaseItems.Add(new PurchaseItem(7, Purchases[1], Items[1]));
    PurchaseItems.Add(new PurchaseItem(8, Purchases[2], Items[1]));
    PurchaseItems.Add(new PurchaseItem(9, Purchases[2], Items[0]));
    PurchaseItems.Add(new PurchaseItem(10, Purchases[2], Items[4]));
  }

  public void Dispose()
  {
    //Do nothing
  }
}


public class Item
{
  public int ID { get; set; }
  public string Name { get; set; }

  public IList<PurchaseItem> PurchaseItems { get; set; }

  public Item()
  {
    PurchaseItems = new List<PurchaseItem>();
  }
}

public class Purchase
{
  public int ID { get; set; }
  public string Name { get; set; }
  public DateTime? ConfirmedDate { get; set; }
  public bool IsValid { get; set; }

  public IList<PurchaseItem> PurchaseItems { get; set; }

  public Purchase()
  {
    PurchaseItems = new List<PurchaseItem>();
  }
}

public class PurchaseItem
{
  public int ID { get; set; }

  public int PurchaseId { get; set; }
  public Purchase Purchase { get; set; }

  public int ItemId { get; set; }
  public Item Item { get; set; }

  public PurchaseItem(int id, Purchase purchase, Item item)
  {
    ID = id;

    PurchaseId = purchase.ID;
    Purchase = purchase;
    purchase.PurchaseItems.Add(this);

    ItemId = item.ID;
    Item = item;
    item.PurchaseItems.Add(this);
  }
}