从linq检索重复值 [英] retrieve duplicate value from linq

问题描述

大家好，

我对linq语句有一些疑问.这是我的桌子，名为Tblactivity

activityid bigint
studentid bigint
activitydate datetime
说明nvarchar(Max)
状态位

我用数据填充Tblactivity.如

activityid |学生证|活动日期|描述|状态
1 | 1 | 2012-08-15 00:30:00 | descA | A
2 | 3 | 2012-08-16 12:30:00 | descAA | A
3 | 2 | 2012-08-16 13:15:12 | descB | B
4 | 3 | 2012-09-02 10:20:11 |说明| A
5 | 3 | 2012-09-02 11:00:00 |说明| A
6 | 4 | 2012-09-02 14:00:00 | descAA | S

所以我开发linq以获得学生ID = 3的详细信息.这是我的linq代码

Hi all,

I have some issue with linq statement. here is my table, named Tblactivity

activityid bigint
studentid bigint
activitydate datetime
description nvarchar(Max)
status bit

i populate Tblactivity with data. such as

activityid | studentid | activitydate | description | status
1 | 1 | 2012-08-15 00:30:00 | descA | A
2 | 3 | 2012-08-16 12:30:00 | descAA | A
3 | 2 | 2012-08-16 13:15:12 | descB | B
4 | 3 | 2012-09-02 10:20:11 | descS | A
5 | 3 | 2012-09-02 11:00:00 | descS | A
6 | 4 | 2012-09-02 14:00:00 | descAA | S

so i develop linq to get detail information of student id = 3. here is my linq code

public List<Tblactivity> getStudents(long studentid, string status)
   {
	studentdatacontext db=new studentdatacontext;
            try
            {
                List<Tblactivity> BG = (from o in db.Tblactivities
                              orderby o.activitydate descending where o.studentid == studentid &&  
                              o.activitydate.Value.Month <= DateTime.Now.Month &&
                              o.activitydate.Value.Month >= DateTime.Now.Date.AddMonths(-1)&&
                              o.status == status select o).ToList();

                return BG;
            }
            catch (Exception e)
            {
                throw e;
            }
        }

所以我根据学生ID = 3获得了所有记录，但是当活动日期相同(最新记录)时，我只想要一条记录.
我读了一些文章，他们建议我使用MAX().但我不知道如何发展以适应上述功能.请建议我.

谢谢，
df

so i got all of the records according to student id = 3.but i want only one record when the activitydate is same (the latest record).
i read some article they suggest me to use MAX(). but i don''t know how to develop to suit with the above function. pls suggest me.

Thanks,
df

推荐答案

hiii，

如果您的活动日期的数据类型为C#DateTime，则

使用以下查询

hiii,

if your activitydate is of datatype C# DateTime then

use following query

List<Tblactivity> BG= (from o in db.Tblactivities
                                     where o.studentid == studentid &&
                                       o.activitydate.Value.Month <= DateTime.Now.Month &&
                                       o.activitydate.Value.Month >= DateTime.Now.Date.AddMonths(-1) &&
                                       o.status == status
                                       && o.activitydate == (from n in db.Tblactivities where n.studentid == o.studentid && Convert.ToString(n.activitydate).Substring(0, 10) == Convert.ToString(o.activitydate).Substring(0, 10) orderby n.activitydate descending select n).Max(l => l.activitydate)
                                   select o).Distinct().ToList();

您知道这只是一条记录为了得到，为什么不使用First()而不是ToList()进行检索，因为您要按Activitydate对其进行降序排序，所以第一条记录将是您的最新记录.

You know it is only one record you need to get, why don''t you retrieve using First() instead of ToList() since you are sorting it descending by activitydate, the first record is going to be your latest record.

试试这个:

Try this:

List<tblactivity> BG = (from o in db.Tblactivities
                       orderby o.activitydate descending where o.studentid == studentid &&  
                       o.activitydate.Value.Month <= DateTime.Now.Month &&
                       o.activitydate.Value.Month >= DateTime.Now.Date.AddMonths(-1)&&
                       o.status == status select o).ToList().Distinct();

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