使用子查询将 SQL 转换为 LINQ

Question

我正在努力将以下 SQL 转换为 LINQ。谁能帮帮我：

   
 SELECT M.Id, M.borrower_id, M.application_id,  [borrower_first_name] ,[borrower_last_name], DP.ID, DP.masterline_id, PR.ID, PR.masterline_id
  FROM [dbo].[CCMemberizationPII] AS M
         LEFT JOIN 
          (SELECT  MAX(DP.ID) ID, DP.masterline_id
          FROM [dbo].[CCDailyPositions] AS DP 
          GROUP BY DP.masterline_id) DP
       ON DP.masterline_id = M.application_id
        LEFT JOIN 
          (SELECT  MAX(PR.ID) ID, PR.masterline_id
          FROM [dbo].[CCPurchase] AS PR
          GROUP BY PR.masterline_id) PR
       ON PR.masterline_id = M.application_id 
  order by M.Id 

Answer 1

在构建 LINQ 查询时，尝试分解部分以构建最终查询。

var positions = 
    from dp in ctx.CCDailyPositions
    group by new { dp.masterline_id } into g
    select new 
    {
        ID = g.Max(x => x.ID),
        dp.masterline_id
    };

var purchases = 
    from pr in ctx.CCPurchase
    group by new { pr.masterline_id } into g
    select new 
    {
        ID = g.Max(x => x.ID),
        pr.masterline_id
    };

var query = 
    from m in ctx.CCMemberizationPII
    from dp in positions.Where(dp => dp.masterline_id == m.application_id)
        .DefaultIfEmpty()
    from pr in purchases.Where(pr => pr.masterline_id == m.application_id)
        .DefaultIfEmpty()
    orderby m.Id
    select new 
    {
        m.Id, 
        m.borrower_id, 
        m.application_id,  
        m.borrower_first_name,
        m.borrower_last_name, 
        DP_ID = dp.ID, 
        PR_ID = pr.ID, 
    };

您的查询也可以使用OUTER APPLY 编写。请注意，EF Core 可以使用窗口函数将 OUTER APPLY 转换为 LEFT JOIN。

var query = 
    from m in ctx.CCMemberizationPII
    from dp in ctx.CCDailyPositions.Where(dp => dp.masterline_id == m.application_id)
        .OrderByDescending(dp => dp.ID)
        .Take(1)
        .DefaultIfEmpty()
    from pr in ctx.CCPurchase.Where(pr => pr.masterline_id == m.application_id)
        .OrderByDescending(pr => pr.ID)
        .Take(1)
        .DefaultIfEmpty()
    orderby m.Id
    select new 
    {
        m.Id, 
        m.borrower_id, 
        m.application_id,  
        m.borrower_first_name,
        m.borrower_last_name, 
        DP_ID = dp.ID, 
        PR_ID = pr.ID, 
    };

哪种 SQL 更好，取决于您特定情况下的执行计划。