在 LINQ 查询连接中查找最大日期

Question

请原谅我对这个问题缺乏了解，因为这个问题非常基本，但我有一个可为空 [DateTime] 列，我想使用 LINQ 查询为每个用户选择记录通过连接找到的最大“时间戳”。

这是我现在的疑问。

return (from t1 in db.BU
                from t2 in db.UserNames
                 .Where(t => (t.User_Username == t1.Username))
                from t3 in db.Logins
                .Where(t => (t.username == t1.Username))
                where myBusinessUnits.Contains(t1.BusinessUnit_ID)
                orderby (t2.Name) descending
                select new GlobalMyTeamDetailModel
                {
                     LastLogin = t3.timestamp,
                     Name = t2.Name

                });

如果有人能帮助我，我将不胜感激。

这是我试图为每个用户获取的“登录”表的最大值。

编辑：另一次尝试没有给出期望的结果：

        var result =
           (from t1 in db.LoginHistories
            where t1.username == user

            from t2 in db.UserNames
                .Where(t => (t.User_Username == t1.username))

            from t3 in db.UserBusinessUnits
                .Where(t => (t.Username == t1.username))

            where myBusinessUnits.Contains(t3.BusinessUnit_ID)

            group t1 by new { t1.username } into g
            let MaxDate = g.Max(uh => uh.timestamp)

            select new GlobalMyTeamDetailModel
                {
                    LastLogin = MaxDate,
                    Name = g.Key.username
                });

Answer 1

如果您始终至少有一个连接行，请按时间戳对 t3 进行排序并获取第一个（应该是最大值）。

return (from t1 in db.BU
            from t2 in db.UserNames
             .Where(t => (t.User_Username == t1.Username))
            from t3 in db.Logins
            .Where(t => (t.username == t1.Username))
            .OrderBy(u => u.timestamp).First()           // <--------- add this
            where myBusinessUnits.Contains(t1.BusinessUnit_ID)
            orderby (t2.Name) descending
            select new GlobalMyTeamDetailModel
            {
                 LastLogin = t3.timestamp,
                 Name = t2.Name

            });

Answer 2

在 linq 中使用 group by 和 max 如下

 From p In db.Products _
            Group p By p.CategoryID Into g = Group _
            Select New With {g, .MaxPrice = g.Max(Function(p) p.UnitPrice)}