Linq 左连接返回内连接

Question

我正在尝试使用 Linq 在 C# 中执行外连接，指导我的人一直说我不应该尝试执行外连接，这并不是真正的答案。

我从其他线程得到的是，我需要.DefaultIfEmpty()，而我可能没有记录。

我首先在可能缺少信息的行上进行了尝试，然后将其添加到每一行以查看是否是问题所在。

每次我运行它时，我只得到内部连接记录。除了不包括我的数据库中仅在前两个表中包含信息的两条记录之外，它的效果很好。

var sqlQuery =
from s in ctx.Suppliers
from sp in ctx.SupplierParts
    .Where(sp => sp.SupplierID == s.SupplierID)
    .DefaultIfEmpty()
from sm in ctx.SupplierManufacturerRelations
    .Where(sm => sm.SupplierPNID == sp.SupplierPNID)
    .DefaultIfEmpty()
from mp in ctx.ManufacturerParts
    .Where(mp => mp.MfgPNID.Equals(sm.MfgPNID))
    .DefaultIfEmpty()
from m in ctx.Manufacturers
    .Where(m => m.ManufacturerID.Equals(mp.ManufacturerID))
    .DefaultIfEmpty()
from im in ctx.ItemMasters
    .Where(im => im.PreID == mp.PreID)
    .Where(im => im.PartNumber == mp.PartNumber)
    .DefaultIfEmpty()
from c in ctx.ComponentClasses
    .Where(c => c.CCID == im.CCID)
    .DefaultIfEmpty()
from um in ctx.UnitsOfMeasures
    .Where(um => um.UOMID == sp.UOMID)
    .DefaultIfEmpty()

select new
{ my variables}

var querylist = sqlQuery.Where(n => n.SupplierID == thisSupplier).ToList();

我也试过

from s in ctx.Suppliers
    join sp in ctx.SupplierParts on s.SupplierID equals sp.SupplierID
    join sm in ctx.SupplierManufacturerRelations on sp.SupplierPNID equals sm.SupplierPNID into spartgroup
from sm in spartgroup.DefaultIfEmpty()
    join mp in ctx.ManufacturerParts on sm.MfgPNID equals mp.MfgPNID into mpartgroup
from mp in mpartgroup.DefaultIfEmpty()
     join m in ctx.Manufacturers on mp.ManufacturerID equals m.ManufacturerID into mgroup
from m in mgroup.DefaultIfEmpty()
     join im in ctx.ItemMasters 
     on new { key1 = (int)mp.PreID, key2 = (int)mp.PartNumber }
     equals new { key1 = im.PreID, key2 = im.PartNumber }
     into tpartgroup
from im in tpartgroup.DefaultIfEmpty()
     join c in ctx.ComponentClasses on im.CCID equals c.CCID into fullgroup
from c in fullgroup.DefaultIfEmpty()
     join um in ctx.UnitsOfMeasures on sp.UOMID equals um.UOMID

此 SQL 查询有效且不会省略行

    SELECT Supplier.SupplierID
         , SupplierPart.SupplierPNID
         , SupplierPart.SupplierPN
         , SupplierPart.Description
         , SupplierManufacturerRelation.MfgPNID
         , ManufacturerPart.PreID
         , ManufacturerPart.PartNumber
         , ItemMaster.CCID
         , ItemMaster.Description AS Expr1
      FROM Supplier  
Inner JOIN SupplierPart 
        ON Supplier.SupplierID = SupplierPart.SupplierID 
 Left JOIN SupplierManufacturerRelation 
        ON SupplierPart.SupplierPNID = SupplierManufacturerRelation.SupplierPNID 
 Left JOIN ManufacturerPart 
        ON SupplierManufacturerRelation.MfgPNID = ManufacturerPart.MfgPNID 
 Left JOIN ItemMaster 
        ON ManufacturerPart.PreID = ItemMaster.PreID 
       AND ManufacturerPart.PartNumber = ItemMaster.PartNumber
     WHERE Supplier.SupplierID = 9

Answer 1

用于将 SQL 转换为 LINQ 查询理解：

1. 将 FROM 子选择转换为单独声明的变量。
2. 按 LINQ 子句顺序翻译每个子句，将一元运算符（DISTINCT、TOP 等）翻译成应用于整个 LINQ 查询的函数。
3. 使用表别名作为范围变量。使用列别名作为匿名类型字段名称。
4. 对多个列使用匿名类型 (new { ... })。
5. Left Join 是通过使用 into join_variable 来模拟的，然后从 join 变量中执行另一个 from，然后是 .DefaultIfEmpty()。
6. 将COALESCE 替换为条件运算符和空测试。
7. 将IN 转换为.Contains() 和NOT IN 转换为!...Contains()
8. SELECT * 必须替换为 select range_variable 或对于连接，一个包含所有范围变量的匿名对象。
9. SELECT 字段必须替换为 select new { ... } 创建一个包含所有所需字段或表达式的匿名对象。
10. 必须使用扩展方法处理正确的FULL OUTER JOIN。

所以从您的 SQL 来看，您的查询应该如下所示：

var ans = from s in ctx.Suppliers
          join sp in ctx.SupplierParts on s.SupplierID equals sp.SupplierID
          join sm in ctx.SupplierManufacturerRelations on sp.SupplierPNID equals sm.SupplierPNID into smj
          from sm in smj.DefaultIfEmpty()
          join mp in ctx.ManufacturerParts on sm?.MfgPNID equals mp.MfgPNID into mpj
          from mp in mpj.DefaultIfEmpty()
          join im in ctx.ItemMasters on new { key1 = (int)mp.PreID, key2 = (int)mp.PartNumber } equals new { key1 = im.PreID, key2 = im.PartNumber } into imj
          from im in imj.DefaultIfEmpty()
          select new {
              s.SupplierID, sp.SupplierPNID, sp.SupplierPN, sp.Description, sm.MfgPNID, mp.PreID, mp.PartNumber, im.CCID, Expr1 = im.Description
          };