在 Linq 中使用计数返回与 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

SQL

谁能帮我修好？

Answer 1

SQL 查询很可能可以在没有group by 的情况下在 LINQ 中重写，但可以使用简单的 LINQ GroupJoin。

但是转换的主要问题是 SQL COUNT(expr) 不计算 NULL 值，并且没有直接的 LINQ 等效项，因此需要条件 Count 或 Sum（我个人更喜欢后者因为通常它会转化为更好的 SQL）。

因此，您查询所需的最小更改是

group t by new 到 group t0 by new

和SoLuong =(Int32)g.Count() 到SoLuong = g.Sum(t0 => t0 != null ? 1 : 0)

附：如开头所述，我将尝试以下 LINQ 查询：

var queryChuyenNganh = 
    from t in myPhanLoaiTaiLieuDataContext.ChuyenNganhs
    join bb in myPhanLoaiTaiLieuDataContext.BaiBaos
    on t.ChuyenNganhID equals bb.ChuyenNganhID into t_BaiBaos
    select new
    {
        t.ChuyenNganhID,
        t.TenChuyenNganh,
        SoLuong = t_BaiBaos.Count()
    }; 

Answer 2

不同之处在于 SQL 对外部连接表中的列 BaiBao.ChuyenNganhID 中的非空值进行计数，而 LINQ 对所有记录进行计数。

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)
   };