How to find the latest data in the record!

Data
Known data:

column1    column2     column3    Column4A                       10                       am         1999-01-01 00:00:00.000a                       11                       am         1999-01-02 00:00:00.000b                      12                       BM          1999-01-03 00:00:00.000b                      13                       bm          1999-01-04 00:00:00.000c                     14                       cm          1999-01-05 00:00:00.000c                      15                      cm        &nbsp 1999-01-06 00:00:00.000

Request data:

column1    column2     column3    Column4A                       11                       am         1999-01-02 00:00:00.000b                      13                       bm         1999-01-04 00:00:00.000c                      15                      cm          1999-01-06 00:00:00.000

--Data loading

Create Table #T (Column1 varchar), Column2 int,column3 varchar (a), Column4 datetime) Insert #T Select ' A ', ' am ', ' 1999-1-1 ' UNION ALL Select ' A ', one, ' am ', ' 1999-1-2 ' union ALL select ' B ', one, ' bm ', ' 1999-1-3 ' union ALL select ' B ', ', ' BM ', ' 1999-1-4 ' UNION ALL SELECT ' C ', M, ' cm ', ' 1999-1-5 ' union ALL SELECT ' C ', ' cm ', ' 1999-1-6 '

--Test Statement Method 1:

Select A.* from #T a WHERE (A.COLUMN4) = (select top 1 (Column4) to #T where Column1 = a.column1 ORDER by Column4 des C

--Test results:

column1    column2     column3    Column4                                                  -------------------------------------------------------------- -----------------------a          11           am         1999-01-02 00:00:00.000b           13          bm          1999-01-04 00:00:00.000c           15          cm          1999-01-06 00:00:00.000

--Test Statement Method 2: (Best efficiency)

Select A.* from #T a join (select Column1,column4=max (COLUMN4) to #T Group by Column1) B on A.column1=b.column1 and a.co Lumn4=b.column4order by A.column1

--Test Results Method 2:

column1    column2     column3    Column4                                                  -------------------------------------------------------------- -----------------------a          11           am         1999-01-02 00:00:00.000b           13          bm          1999-01-04 00:00:00.000c           15          cm          1999-01-06 00:00:00.000