SQL：在Linq中從x左聯接y中選擇SELECT id，MAX（y.DateCreated）嗎？

Question

我似乎無法將此SQL重寫為EF Linq：

SELECT      Conversation.Id
FROM        Conversation LEFT JOIN Message
                ON  Conversation.Id = Message.ConversationId
GROUP BY    Conversation.Id
ORDER BY    MAX(Message.DateCreated) DESC

我認為這樣的事情會起作用：

 _dbContext.Conversation
           .OrderByDescending(c => c.Messages.DefaultIfEmpty().Max(m => m.DateCreated))
           .Select(cm => cm.Id);

但這給了我錯誤System.InvalidOperationException : Sequence contains no elements.

還有這個：

_dbContext.Conversation
          .Select(c => new {c.Id, MaxDate = c.Messages.DefaultIfEmpty().Max(m => m.DateCreated)})
          .OrderByDescending(c => c.MaxDate)
          .Select(cm => cm.Id);

但這給了我System.ArgumentException : At least one object must implement IComparable. 。

正確的方法是什么？

Answer 1

你可以嘗試一下：

from r in Conversation
              join ru in Message
              on r.Id equals ru.ConversationId into ps
              from ru in ps.DefaultIfEmpty()
              group ru by new { ru.ConversationId, ru.DateCreated } into rug
              select new {
                  id = ru.ConversationId,
                  datecreated = rug.Max(ru => ru.datecreated)
              }).OrderByDescending(x => x.datecreated);

這可能無法編譯，因為我沒有用它進行測試的代碼（像小提琴一樣）

Answer 2

不過，您已經關閉，只需刪除DefaultIfEmpty

_dbContext.Conversation.Select(con => new 
    {
        con.Id,
        MaxDateCreated = (DateTime?) con.Messages.Max(msg => msg.DateCreated)
    })
.OrderByDescending(con => con.MaxDateCreated)
.ToArray()

這是將要生成的

SELECT 
[Project2].[C1] AS [C1], 
[Project2].[Id] AS [Id], 
[Project2].[C2] AS [C2]
FROM ( SELECT 
    [Project1].[Id] AS [Id], 
    1 AS [C1], 
     CAST( [Project1].[C1] AS datetime2) AS [C2]
    FROM ( SELECT 
        [Extent1].[Id] AS [Id], 
        (SELECT 
            MAX([Extent2].[DateCreated]) AS [A1]
            FROM [dbo].[Message] AS [Extent2]
            WHERE [Extent1].[Id] = [Extent2].[ConversationId]) AS [C1]
        FROM [dbo].[Conversation] AS [Extent1]
    )  AS [Project1]
)  AS [Project2]
ORDER BY [Project2].[C2] DESC

Answer 3

我相信這將生成您要查找的SQL（加上或減去嵌套的子查詢）：

var ans = from c in Conversation
          join m in Message on c.Id equals m.ConversationId into mj
          from m in mj.DefaultIfEmpty()
          group m by c.Id into mg
          orderby mg.Max(m => m.DateCreated) descending
          select mg.Key;

但是，在LINQ中，可以使用組聯接而不是常規聯接。 這也應該做您想要的，但是在SQL中使用子選擇，因此我不確定哪個在服務器上更有效。 普遍的想法似乎是加入更好，但是像現代足夠的優化程序那樣，逐個加入可能會使它變得毫無意義。 我發現的另一篇文章說，子查詢（CIS）效率更高，所以可能更好。

var ans2 = from c in Conversation
           join m in Message on c.Id equals m.ConversationId into mj
           orderby mj.Max(m => m.DateCreated) descending
           select c.Id;