T-SQL - SELECT by nearest date and GROUPED BY ID

Question

From the data below I need to select the record nearest to a specified date for each Linked ID using SQL Server 2005:

ID     Date      Linked ID
...........................
1    2010-09-02     25
2    2010-09-01     25
3    2010-09-08     39
4    2010-09-09     39
5    2010-09-10     39
6    2010-09-10     34
7    2010-09-29     34
8    2010-10-01     37
9    2010-10-02     36
10   2010-10-03     36

So selecting them using 01/10/2010 should return:

1    2010-09-02     25 
5    2010-09-10     39
7    2010-09-29     34 
8    2010-10-01     37
9    2010-10-02     36

I know this must be possible, but can't seem to get my head round it (must be too near the end of the day :P) If anyone can help or give me a gentle shove in the right direction it would be greatly appreciated!

EDIT: Also I have come across this sql to get the closest date:

abs(DATEDIFF(minute, Date_Column, '2010/10/01'))

but couldn't figure out how to incorporate into the query properly...

Thanks

Solution 1

you can try this.

DECLARE @Date DATE = '10/01/2010';

WITH cte AS
    (
    SELECT ID, LinkedID, ABS(DATEDIFF(DD, @date, DATE)) diff,
        ROW_NUMBER() OVER (PARTITION BY LinkedID ORDER BY ABS(DATEDIFF(DD, @date, DATE))) AS SEQUENCE
    FROM MyTable
    )

SELECT *
FROM cte
WHERE SEQUENCE = 1
ORDER BY ID
;

You didn't indicate how you want to handle the case where multiple rows in a LinkedID group represent the closest to the target date. This solution will only include one row And, in this case you can't guarantee which row of the multiple valid values is included.

You can change ROW_NUMBER() with RANK() in the query if you want to include all rows that represent the closest value.

Solution 2

You want to look at the absolute value of the DATEDIFF function (http://msdn.microsoft.com/en-us/library/ms189794.aspx) by days.

The query can look something like this (not tested)

with absDates as 
(
   select *, abs(DATEDIFF(day, Date_Column, '2010/10/01')) as days
   from table
), mdays as
( 
   select min(days) as mdays, linkedid
   from absDates
   group by linkedid
)
select * 
from absdates
inner join mdays on absdays.linkedid = mdays.linkedid and absdays.days = mdays.mdays

Comments

[Iain Ward]

@Hogan Ah yes I have come across it already, but forgot to mention that in the question, so I've updated it. Thanks for mentioning that

[Hogan]

@w69rdy : example query added.

[Hogan]

this is nicer than my query (only one select) but mine may be clearer to a beginner...

[Sean Reilly]

The example query might not be right -- it should be min(days), otherwise you would return the largest difference, right? Also, I don't think that the performance on this will be very good at all. In general I would recommend using the ROW_NUMBER() solution. It should be more straightforward and more performant.

[bobs]

@Hogan, your query fails right now. Where is table absdays defined?

[Sean Reilly]

More information on ROW_NUMBER() is available here: msdn.microsoft.com/en-us/library/ms186734.aspx

[Hogan]

@bobs : It wasn't :D I fixed the typo.

[Hogan]

@Sean : I changed the max to min -- and I agree that Row_Number() will be more performant. I don't agree that it is clearer since it is a MS only solution and not part of standard SQL. Of course that does ring hollow since I'm using CTEs.

[Hogan]

Wow, this is def. least performant -- a select for every linkedid.

[Iain Ward]

+1 @Hogan Thanks for your answer, it works but I think I prefer bobs as its a little bit slicker and removes duplicates :)

[pcofre]

Yes, it´s just for small tables

[Thomas]

@Hogan - I'm not sure I agree. If you are going to use a CTE anyway, then you might as well take advantage of Row_Number().

[Iain Ward]

Thanks for your answer. Duplicates are irrelevant, as long as it returns one thats all I need

[Hogan]

@Thomas : Yeah, if you seem my comment on my answer to @Sean I basically said as much (and voted for your answer.)

[Hogan]

@w69rdy : yep, it is a better TSQL answer. This is the way to do it if you don't have TSQL (and used temp tables instead of CTE

[Sean Reilly]

@Hogan: it's worth nothing that if temp tables were used, performance probably wouldn't be a problem. I would say this might be the preferred solution for databases other than mssql 2005 and later.

[Hogan]

@Sean : I agree unless that SQL includes functionally that allows for Ranking functionality.