這些查詢爲什麼會生成不同的sql？

Question

我在實體框架中的linq查詢有問題。我正在查詢導航屬性上的某個字段。問題是生成的sql不夠理想。下面的例子被簡化了，實際上我試圖傳遞一個表達式樹，這就是爲什麼使用let綁定的第二個查詢不是一個足夠的解決方案，即使生成的sql是我想要的。

所以總結我有兩個問題：

爲什麼生成的sql不同？有沒有什麼辦法來產生一個SQL查詢不會創建每個標準與表達式樹的聯接？

更新：我意識到，我必須包含（「證券」）的第一個查詢，而不是第二次，當我第一次張貼的問題，但它不會改變審覈規定的應用方式，僅列選擇。

var qry = db.Positions     
    .Where(criteria) 
    .ToList(); 

var qry1 = (from p in db.Positions 
      where p.Security.Country == "NO" || p.Security.Country == "US" || p.Security.Country == "GB" 
      select p).ToList(); 

var qry2 = (from p in db.Positions 
      let s = p.Security 
      where s.Country == "NO" || s.Country == "US" || s.Country == "GB" 
      select p).ToList(); 


--qry1 
SELECT 
    [Extent1].* --All columns from tblPositions 
    FROM  [dbo].[tblPositions] AS [Extent1] 
    LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent2] ON ([Extent2].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent2].[SecuritySeq]) 
    LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent3] ON ([Extent3].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent3].[SecuritySeq]) 
    LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent4] ON ([Extent4].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent4].[SecuritySeq]) 
    LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent5] ON ([Extent5].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent5].[SecuritySeq]) 
    WHERE [Extent2].[Country] = 'NO' OR [Extent3].[Country] = 'US' OR [Extent4].[Country] = 'GB' 

--qry2 
SELECT 
    [Extent1].* 
    FROM [dbo].[tblPositions] AS [Extent1] 
    LEFT OUTER JOIN [dbo].[tblSecurities] AS [Extent2] ON ([Extent2].[SecurityType] IN (1,2..)) AND ([Extent1].[Security] = [Extent2].[SecuritySeq]) 
    WHERE [Extent2].[Country] IN ('NO','US','GB') 

Answer 1

在第一個查詢

var qry1 = (from p in db.Positions.Include("Security") 
      where p.Security.Country == "NO" 
      || p.Security.Country == "US" 
      || p.Security.Country == "GB" 
      select p).ToList(); 

你似乎認爲實體框架具有神奇的力量，可以推斷，每個所生成的表達式樹的or報表是同一個對象，並它可以將它們組合成一個包含。到目前爲止，這並不是那麼聰明。

此外，兩個查詢返回的行都不相同。第二個也需要include，以包含來自安全的行（如果需要的話）。否則，您可以從第一個查詢中刪除它們（僅包含返回行，它與where子句篩選行的能力無關）。

或者使其更加面向對象和可讀性。

var allowedCountries = new List<string>() { "NO, "US", "GB" }; 

var qry1 = (from p in db.Positions 
      // I'm not sure if this is exactly correct 
      where p.Security.Country in allowedCountries 
      select p).ToList(); 

或拉姆達（我更熟悉）

var qry1 = db.Positions 
      .Where(p => allowedCountries.Contains(p.Security.Country)) 
      .ToList();