Get distinct max date using SQL

I'm not sure quite how to title my question but this is what I'm trying to do:

Given pc_tmppl_tbl

pc_tmppl_attach pc_tmppl_val1    pc_tmppl_crtdt

AJC05-06    AJCINT       2005-08-15 10:32:03.790
AJC06-07    AJCINT       2006-10-17 10:02:06.570
AJC07-08    AJCINT       2007-06-13 10:44:53.573
AJC08-09    AJCINT       2008-06-27 09:51:17.290
AJC09-10    AJCINT       2009-07-20 14:26:06.270
AJC10-11    AJCINT       2010-08-26 11:54:32.777
AJC99-001   AJCINT       2005-05-30 19:30:51.623
ALPI05-06   ALPINE       2005-05-30 19:30:51.623
ALPI07-08   ALPINE       2006-12-11 13:57:09.923
ALPI07-08   ALPINE       2007-05-24 14:04:07.867
ALPI08-09   ALPINE       2008-04-30 09:49:24.140

I want it to return the max date for pc_tmppl_crtdt and its corresponding pc_tmppl_attach so

ALPI08-09   ALPINE       2008-04-30 09:49:24.140
AJC10-11    AJCINT       2开发者_运维技巧010-08-26 11:54:32.777

I've been trying sub queries but haven't quite hit on the answer, any help would be much appreciated.

Select T.pc_tmppl_attach, T.pc_tmppl_val1, T.pc_tmppl_crtdt
From pc_temppl_tbl As T
    Join    (
            Select pc_tmppl_val1, Max( T1.pc_tmppl_crtdt ) As MaxDateTime
            From pc_temppl_tbl As T1
            Group By T1.pc_tmppl_val1
            ) As Z
        On Z.pc_tmppl_val1 = T.pc_tmppl_val1
            And Z.MaxDateTime = T.pc_tmppl_crtdt

You didn't define what server and version you're using - if you're on SQL Server 2005 or newer, you can use a CTE (Common Table Expression) and a ranking function - something like this:

;WITH PartitionedData AS
(
   SELECT 
       pc_tmppl_attach, pc_tmppl_val1, pc_tmppl_crtdt,
       ROW_NUMBER() OVER(PARTITION BY pc_tmppl_val1 
                         ORDER BY pc_tmppl_crtdt DESC) AS 'RowNumber'
  FROM dbo.pc_tmppl_tbl
)
SELECT
   pc_tmppl_attach, pc_tmppl_val1, pc_tmppl_crtdt
FROM
   PartitionedData
WHERE
   RowNumber = 1

Basically, what the CTE (inner select) does is grab all data from your table, partition it by your column pc_tmppl_val1 - so each group of values for pc_tmppl_val1 starts counting back at 1 - and order those entries by pc_tmppl_crtdt descending - newest entry is first.

So for each pc_tmppl_val1 value, the newest entry is the entry with the RowNumber = 1 and that's what the outer SELECT (based on the CTE) gives you.

The CTE and ranking function are very flexible, too - if you need to top 3 entries for each pc_tmppl_val1 value, just change the outer WHERE condition to

WHERE RowNumber <= 3

and you're done!

The CTE (Common Table Expression) and ranking functions are ANSI SQL standard - so other databases besides Microsoft SQL Server support it, too (I just know SQL Server the best - that's why I use it as a sample).

Using a correlated query (which should work with most sql databases):

SELECT pc_tmppl_attach,pc_tmppl_val1, pc_tmppl_crtdt
FROM pc_tmppl_tbl AS tbl_ext
WHERE pc_tmppl_crtdt = (
                        SELECT MAX(tbl_int.pc_tmppl_crtdt)
                        FROM pc_tmppl_tbl AS tbl_int
                        WHERE tbl_int.pc_tmppl_val1 = tbl_ext.pc_tmppl_val1
                       )

SELECT TOP 1 [pc_tmppl_attach], [pc_tmppl_crtdt]
FROM [Given pc_tmppl_tbl]
order by pc_tmppl_crtdt desc