linq中的日期时间

Question

所以我试图从脚本任务内的表中获取最新记录。错误出现在 Linq 查询中，如下所示：

"Could not Translate expression 'Table(SSASLogging).Select(r=>r.TimeStamp).Max()' into SQL and could not treat it as a local expression."

问题在于 DateTime 数据类型，但我需要 DateTime 将其提供给我的 SSIS 变量。我知道这可以在执行 SQL 任务中轻松完成，但我现在放弃太远了！我知道 DateTime 有一些 LinqToSQL 方法，但它们看起来是为了比较，我不知道如何在这里应用它们。

    public DateTime getLatest()
    {
        DateTime result = new DateTime();

        //temp dummy/defaul date is two days ago
        result = DateTime.Now.AddDays(-2);

        try
        {
            //get the data connection string from the connection manager
            RW = (string)Dts.Connections["ReportingWarehouse"].ConnectionString;

            //Remove the Provider, Auto Translate, and Application
            //as it is not a parameter for the DataContext constructor
            RW = RW.Remove(RW.IndexOf("Provider=SQLNCLI10.1;"), "Provider=SQLNCLI10.1;".Length);
            RW = RW.Remove(RW.IndexOf("Auto Translate=False;"), "Provider=SQLNCLI10.1;".Length);
            RW = RW.Remove(RW.IndexOf("Application"),RW.Length - RW.IndexOf("Application"));



            MessageBox.Show(RW);

            //get the last insertion date from the SSASLoging table
            using (DataContext RWData = new DataContext(RW))
            {
                Table<SSASLogging> records = RWData.GetTable<SSASLogging>();
                var rs = (from r in records
                         select r.TimeStamp).Max();
                //result = rs.FirstOrDefault();
            }

        }

        catch (Exception e)
        {
            MessageBox.Show("Exception in Retrieving latesttime" + e.Message + "/n"
                            + e.StackTrace);
        }

        return result;
   }

}//end partial class


[Table]  
public class SSASLogging  
{
    [Column(Name = "CREATED_TIMESTAMP")] 
    private DateTime timeStamp;

    public DateTime TimeStamp 
    {
        get { return this.TimeStamp; }
    }
}//End SSASLogging

Answer 1

如果Max 不起作用，那该怎么办：

var maxDate =
    (from r in records
     orderby r.TimeStamp descending
     select r.TimeStamp)
    .FirstOrDefault();

Answer 2

尝试对表格和Take(1).ToList() 进行排序。它应该返回带有 0 或 1 个元素的 List。然后你可以使用FirstOrDefault，这将由应用程序执行，而不是由SQL：

var rs = (from r in records
          orderby r.TimeStamp descending
          select r).Take(1).ToList().FirstOrDefault();

Answer 3

你可以试试 OrderBy

var rs = (from r in records  orderby r.TimeStamp descending
                     select r).FirstOrDefault();

Answer 4

可能是因为这个：

http://msdn.microsoft.com/en-us/vstudio/ff963710.aspx

所以：

var rs = (from r in records.AsEnumerable()
         select r.TimeStamp).Max();

或：

var rs = (from r in records
         select r.TimeStamp).AsEnumerable().Max();