Get the first row in a group

The following table is the DimWorkItem table, to query the latest work per day for each task

Id	Title	Changeddate	Person	Todayhours
1	Bank card Withholding Agreement signing	2015/3/2 16:34:21	Tom	2
1	Bank card Withholding Agreement signing	2015/3/3 10:14:37	Tom	3
1	Bank card Withholding Agreement signing	2015/3/3 15:03:56	Tom	2.5
2	Password change	2015/3/2 09:03:20	John doe	4
2	Password change	2015/3/3 14:40:51	John doe	2

Select T.id,t.title,t.changeddate,t.person,t.todayhours
From
(Select A.id,a.title,a.person,
CONVERT (varchar), a.changeddate, 111) Changeddate,
A.changeddate,
A.todayhours,

--Grouped by task ID and date changed, sorted by date changed, get first row
Row_number () over (partition by Id,convert (varchar (+), a.changeddate, 111) Order BY changeddate Desc) RK
From Tfs_Warehouse.dbo.DimWorkItem a) t
where T.rk = 1

The query results are as follows:

Id	Title	Changeddate	Person	Todayhours
1	Bank card Withholding Agreement signing	2015/3/2 16:34:21	Tom	2
1	Bank card Withholding Agreement signing	2015/3/3 15:03:56	Tom	2.5
2	Password change	2015/3/2 09:03:20	John doe	4
2	Password change	2015/3/3 14:40:51	John doe	2

Get the first row in a group