10-3. 返回結果是一個標量值

問題

想取得存儲過程返回的一個標量值.

解決方案

假設我們有如Figure 10-2所示的ATM機和ATM機取款記錄的模型

?

Figure 10-2. 一個ATM機和ATM機取款記錄的模型

我們想要用一個存儲過程在指定機器和日期內返回所有的取款總額

Listing 10-9 是這個存儲過程的代碼

Listing 10-9. GetWithdrawals存儲過程，獲取在指定機器和日期內所有的取款總額

create procedure [Chapter10].[GetWithdrawals]

(@ATMId int, @WithdrawalDate date)

as

begin

select SUM(amount) TotalWithdrawals

from Chapter10.ATMWithdrawal

where ATMId = @ATMId and [date] = @WithdrawalDate

end

接下來就是調用這個存儲過程的方法：

1. 右擊模型的設計視圖,選擇“從數據庫更新模型”.在對話框中“存儲過程和函數”下勾選Chapter10/

GetWithdrawals.點擊“完成”.

2.（ 譯注：我的環境是win10+vs2013+ef6.1.3,是不需要這步的，第1步已經把這步也完成了）右擊模型的設計視圖, 選擇“新增“ ? 函數導入. 從“存儲過程/函數名稱”下拉框中選擇GetWithdrawals. 在“函數導入名稱“文本框中輸入：GetWithdrawals. 這個就是在模型中的方法名稱.在“返回以下內容的集合“里勾選“標量”，在下拉框里選擇Decimal.單擊“確定”.

3.接下來用 Listing 10-10所示代碼使用這個GetWithdrawals 存儲過程

Listing 10-10. 用模型GetWithdrawals()方法查詢存儲過程 GetWithdrawals

??? static void Main(string[] args)

??????? {

??????????? DateTime today = DateTime.Parse("5/7/2013");

??????????? DateTime yesterday = DateTime.Parse("5/6/2013");

??????????? using (var context=new EFRecipesEntities())

??????????? {

??????????????? context.Database.ExecuteSqlCommand("delete from chapter10.ATMWithdrawal");

??????????????? context.Database.ExecuteSqlCommand("delete from chapter10.ATMMachine");

??????????????? var atm = new ATMMachine { ATMId = 17, Location = "12th and Main" };

??????????????? atm.ATMWithdrawals.Add(new ATMWithdrawal{Amount= 20.00m,Date=today});

??????????????? atm.ATMWithdrawals.Add(new ATMWithdrawal { Amount = 100.00M, Date = today });

??????????????? atm.ATMWithdrawals.Add(new ATMWithdrawal { Amount = 75.00M, Date = yesterday });

??????????????? atm.ATMWithdrawals.Add(new ATMWithdrawal { Amount = 50.00M, Date = today });

??????????????? context.ATMMachines.Add(atm);

??????????????? context.SaveChanges();

??????????? }

??????????? using (var context=new EFRecipesEntities())

??????????? {

??????????????? var forToday = context.GetWithdrawals(17, today).FirstOrDefault();

??????????????? var forYesterday = context.GetWithdrawals(17, yesterday).FirstOrDefault();

??????????????? var atm = context.ATMMachines.Where(o => o.ATMId == 17).FirstOrDefault();

??????????????? Console.WriteLine("ATMWithdrawals for ATM at {0} at {1}",atm.ATMId,atm.Location);

?????????? ?????Console.WriteLine("\t{0} Total Withdrawn={1}",yesterday.ToShortDateString(),forYesterday.Value.ToString("C"));

??????????????? Console.WriteLine("\t{0} Total Withdrawn={1}",today.ToShortDateString(),forToday.Value.ToString("C"));

??????????? }

?????? ?????Console.WriteLine("\npress any key to exit...");

??????????? Console.Read();

??????? }

輸出結果如下面的 Listing 10-10:

==============================================================

ATM Withdrawals for ATM at 17 at 12th and Main

5/6/2013 Total Withdrawn = $75.00

5/7/2013 Total Withdrawn = $170.00

==============================================================

它是如何工作的？

注意：EF會把從存儲過程取回的標量值認為是一個集合.在我們的例子里,我們的存儲過程返回一個單一的decimal值，我們用FirstOrDefault()方法把它從集合中提取出來

附：創建示例用到的數據庫的腳本文件

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