LINQ Join從第二張表中獲取最新記錄

Question

問題：

它們是兩個表TableEmails和TableSentEmailRecords。 因此，無論何時發送電子郵件，TableSentEmailRecords中都會存儲一條記錄，例如發送時間等。

我想做一個LINQ查詢，它使我得到TableEmails的列，再加上來自TableSentEmailRecords的“ SentDate”列，該列顯示發送電子郵件的最新日期。

我嘗試了這個：

List<CustomEmail> emails = new List<CustomEmail>();

var query = from c in db.TableEmails 
            join o in db.TableSentEmailRecords on c.EmailId equals o.EmailId
                        select new CustomEmail
                        {
                            EmailName = c.EmailName,
                            EmailId= c.EmailId,
                            EmailDescription = c.EmailDescription,
                            LastDateEmailSentOut = o.SentDate
                        };

emails = query.ToList();

但這不一定從TableSentEmailRecords表中獲取該特定電子郵件的最新記錄。

有任何想法嗎 ？？？

Answer 1

您可以只使用嵌套查詢而不是聯接：

var query = from c in db.TableEmails 
            select new CustomEmail
            {
                EmailName = c.EmailName,
                EmailId= c.EmailId,
                EmailDescription= c.EmailDescription,
                LastDateEmailSentOut = db.TableSentEmailRecordson
                                         .Where(x => x.EmailId = c.EmailId)
                                         .Max(x => x.SentDate)
            };

Answer 2

我認為在這種情況下，GroupJoin是正確的方法。 使用子查詢可以工作，但效率不高。 抱歉，我不知道更簡單的查詢語法，尤其是我不知道如何使用它進行組聯接。 也許有人可以翻譯。

db.TableEmails.GroupJoin(db.TableSendEmailRecordson,
  o => o.EmailID,
  i => i.EmailID,
  (o, i) => new
  {
    EmailName = c.EmailName,
    EmailId= c.EmailId,
    EmailDescription= c.EmailDescription,
    LastDateEmailSentOut = i.Max(x => x.SentDate)
  })

要從TableSendEmailRecordson獲取所有詳細信息，您可以執行以下操作：

db.TableEmails.GroupJoin(db.TableSendEmailRecordson,
  o => o.EmailID,
  i => i.EmailID,
  (o, i) => new
  {
    EmailName = c.EmailName,
    EmailId= c.EmailId,
    EmailDescription = c.EmailDescription,
    LastSendEmailRecordson = i.FirstOrDefault(y => y.SentDate == i.Max(x => x.SentDate))
  })

可能會抱怨被翻譯成SQL，如果是這種情況，那么您需要首先獲取數據，然后在查詢中使用結果，例如

var emails = db.TableEmails.ToArray();
var emailSend = db.TableSendEmailRecordson.ToArray();

Answer 3

您可以嘗試這樣的事情：

var query = (from c in db.TableEmails 
            join o in db.TableSentEmailRecordson c.EmailId equals o.EmailId
            select new CustomEmail
            {
                EmailName = c.EmailName,
                EmailId = c.EmailId,
                EmailDescription = c.EmailDescription,
                LastDateEmailSentOut = o.SentDate
            }).OrderByDescending(c => c.LastDateEmailSentOut).ToList();