我在 Visual Studio 2019、C#、ASP.Net Core 中制作多連接查詢時遇到問題。我有兩張桌子：sessions(    id          int primary key,    start_id    int not null,   // foreign key to "start event" event.id    end_id      int             // foreign key to "end event" event.id)events(    id  int primary key,    username    varchar(32) not null,    deviceName  varchar(64) not null,    eventName   varchar(8) not null,    eventDate   datetime not null)登錄/注銷等成對事件被放入事件表中。表的INSERT 觸發器events處理表的管理sessions。對于這個問題，假設會話表可能如下所示： id | start_id | end_id----+----------+--------  1 | 1        | null  2 | 2        | 3events: id | userName | deviceName | eventName | eventDate----+----------+------------+-----------+-----------  1 | alice    | moose      | login     | 2019-03-11 14:02:54  2 | bob      | juno       | login     | 2019-03-11 15:11:08  3 | bob      | juno       | logout    | 2019-03-11 17:18:22在 SSMS 中，我可以將我想要的查詢編寫為：SELECT    sessions.id,    StartEvents.userName,    StartEvents.deviceName,    StartEvents.eventDate as startDate,    ISNULL(EndEvents.eventDate, GETDATE()) as endDateFROM sessionsJOIN events StartEvents    ON sessions.start_id = StartEvents.idLEFT JOIN events EndEvents    ON sessions.end_id = EndEvents.idWHERE StartEvents.eventDate >= @myStart      AND ISNULL(EndEvents.eventDate, GETDATE()) <= @myEnd我在 C# 中試過這個：var result = (    from sessions in db.Sessions    join StartEvents in db.Events        on sessions.Start_id equals StartEvents.Id    join EndEventsTmp in db.Events        on sessions.End_id equals EndEventsTmp.Id into EndEventsTmp2    from EndEvents in EndEventsTmp2.DefaultIfEmpty()    where StartEvents.Machine.Trim().ToUpper().Equals(machine.Trim().ToUpper())          & StartEvents.eventDate >= myStart          & (EndEvents.eventDate ?? DateTime.Now) <= myEnd    orderby StartEvents.Date    }    ).ToList();有人告訴我“'??'的左操作數” 應該是引用或可為空的類型”。問題是基礎字段（事件中的“eventDate”）不能為空。我該如何處理？