在Linq中使用Count会返回与SQL查询不同的结果

Question

我使用Linqer将SQL转换为LinQ，但结果不一样 我的SQL查询：

    Select ChuyenNganh.ChuyenNganhID,ChuyenNganh.TenChuyenNganh,SoLuong= count(BaiBao.ChuyenNganhID )
from BaiBao right join ChuyenNganh on ChuyenNganh.ChuyenNganhID = BaiBao.ChuyenNganhID
group by  ChuyenNganh.ChuyenNganhID, ChuyenNganh.TenChuyenNganh

转换为Linq

var queryChuyenNganh = from t in myPhanLoaiTaiLieuDataContext.ChuyenNganhs
                                   join t0 in myPhanLoaiTaiLieuDataContext.BaiBaos on new { ChuyenNganhID = t.ChuyenNganhID } equals new { ChuyenNganhID = Convert.ToInt32(t0.ChuyenNganhID) } into t0_join
                                   from t0 in t0_join.DefaultIfEmpty()
                                   group t by new
                                   {
                                       t.ChuyenNganhID,
                                       t.TenChuyenNganh
                                   } into g
                                   select new
                                   {
                                       ChuyenNganhID = (System.Int32)g.Key.ChuyenNganhID,
                                       g.Key.TenChuyenNganh,

                                       SoLuong  =(Int32)g.Count()
                                   };

结果：

LINQ的

SQL

谁可以帮我解决？

Answer 1

很可能SQL查询可以在没有//page.Height = XUnit.FromMillimeter(baseY + 10).Point; double height = XUnit.FromMillimeter(baseY + 10).Point; page.CropBox = new PdfRectangle(new XPoint(0, page.Height - height), new XSize(page.Width, height)); 的LINQ中重写，而是简单的LINQ group by。

但转换的主要问题是SQL GroupJoin不计算COUNT(expr)值，并且没有直接的LINQ等效值，因此条件NULL或Count需要（我个人更喜欢后者，因为通常它转换为更好的SQL）。

因此，您查询所需的最小更改是

Sum至group t by new

和group t0 by new到SoLuong =(Int32)g.Count()

P.S。正如开头所提到的，我将尝试以下LINQ查询：

SoLuong = g.Sum(t0 => t0 != null ? 1 : 0)

Answer 2

区别在于SQL在BaiBao.ChuyenNganhID（外连接表中的一列）中计算非空值，而LINQ计算所有记录。

您还需要使LINQ计数非空值：

var queryChuyenNganh = from t in myPhanLoaiTaiLieuDataContext.ChuyenNganhs
   join t0 in myPhanLoaiTaiLieuDataContext.BaiBaos
       on new { t.ChuyenNganhID } equals new { ChuyenNganhID = Convert.ToInt32(t0.ChuyenNganhID) } into t0_join
   from t0 in t0_join.DefaultIfEmpty()
   group new {T=t, NonNull=t0.ChuyenNganhID != null} by new
   {
       t.T.ChuyenNganhID,
       t.T.TenChuyenNganh
   } into g
   select new
   {
       ChuyenNganhID = (System.Int32)g.Key.ChuyenNganhID,
       g.Key.TenChuyenNganh,
       SoLuong  =(Int32)g.Count(x => x.NonNull)
   };