How can I SELECT distinct data based on a date field? - sql

Question

I have table that stores a log of changes to objects in another table. Here are my table contents:
ObjID Color Date User
------- ------- ------------------------ --------
1 Red 2010-01-01 12:22:00.000 Joe
1 Blue 2010-01-02 15:22:00.000 Jill
1 Green 2010-01-03 16:22:00.000 Joe
1 White 2010-01-10 09:22:00.000 Mike
2 Red 2010-01-09 10:22:00.000 Mike
2 Blue 2010-01-12 09:22:00.000 Jill
2 Orange 2010-01-12 15:22:00.000 Joe
I want to select the most recent date for each Object, as well as the Color and User on the date of that record.
Bascically, I want this result set:
ObjID Color Date User
------- ------- ------------------------ --------
1 White 2010-01-10 09:22:00.000 Mike
2 Orange 2010-01-12 15:22:00.000 Joe
I'm having trouble wrapping my head around the SQL query I need to write to get this data...
I am retrieving data via ODBC from an iSeries DB2 database (AS/400).

Answer 1

Hey there, I think you want the following (where ColorTable is your table name):
SELECT Color.*
FROM ColorTable as Color
INNER JOIN
(
SELECT ObjID, MAX(Date) as Date
FROM ColorTable
GROUP BY ObjID
) as MaxDateByColor
ON Color.ObjID = MaxDateByColor.ObjID
AND Color.Date = MaxDateByColor.Date

Answer 2

Assuming at least SQL Server 2005
DECLARE #T TABLE (ObjID INT,Color VARCHAR(10),[Date] DATETIME,[User] VARCHAR(50))
INSERT INTO #T
SELECT 1,'Red',' 2010-01-01 12:22:00.000','Joe' UNION ALL
SELECT 1,'Blue','2010-01-02 15:22:00.000','Jill' UNION ALL
SELECT 1,'Green',' 2010-01-03 16:22:00.000','Joe' UNION ALL
SELECT 1,'White',' 2010-01-10 09:22:00.000','Mike' UNION ALL
SELECT 2,'Red',' 2010-01-09 10:22:00.000','Mike' UNION ALL
SELECT 2,'Blue','2010-01-12 09:22:00.000','Jill' UNION ALL
SELECT 2,'Orange','2010-01-12 15:22:00.000','Joe'
;WITH T AS
(
SELECT *,
ROW_NUMBER() OVER (PARTITION BY ObjID ORDER BY Date DESC) AS RN
FROM #T
)
SELECT ObjID,
Color,
[Date],
[User]
FROM T
WHERE RN=1
Or a SQL Server 2000 method from the article linked to in the comments
SELECT ObjID,
CAST(SUBSTRING(string, 24, 33) AS VARCHAR(10)) AS Color,
CAST(SUBSTRING(string, 1, 23) AS DATETIME ) AS [Date],
CAST(SUBSTRING(string, 34, 83) AS VARCHAR(50)) AS [User]
FROM
(
SELECT ObjID,
MAX((CONVERT(CHAR(23), [Date], 126)
+ CAST(Color AS CHAR(10))
+ CAST([User] AS CHAR(50))) COLLATE Latin1_General_BIN) AS string
FROM #T
GROUP BY ObjID) T;

Answer 3

If you have an Objects table and your ObjectHistory table has an index on ObjID and date, then this could perform better than other queries given so far:
SELECT
X.*
FROM
Objects O
CROSS APPLY (
SELECT TOP 1 *
FROM ObjectHistory H
WHERE O.ObjID = O.ObjID
ORDER BY H.[Date] DESC
) X
The performance improvement may only come if you're pulling columns from the Objects table, too, but it's worth a shot.
If you want all Objects regardless of whether they have a history entry, switch to OUTER APPLY (and of course use O.ObjID instead of H.ObjID).
The neat thing about this query is that
It solves for situations where the Date value can have duplicates
It can support an arbitrary number of items per group (say, the top 5 instead of the top 1)

Answer 4

See these two related questions:
SQL/mysql - Select distinct/UNIQUE but return all columns?
And:
How to efficiently determine changes between rows using SQL

Answer 5

SELECT t1.* FROM Table_name as t1
INNER JOIN (
SELECT MAX(Date) as MaxDate, ObjID FROM Table_name
GROUP BY ObjID
) as t2
ON t1.ObjID = t2.ObjID AND t1.Date = t2.MaxDate

Answer 6

You can find out, per object, its most recent change like this:
select objectid, max(changedate) as LatestChange
from LOG
group by objectid
You can then get the color and user columns by linking the set returned above, instantiated as an inline view that has been given an alias, to the same table again:
select color, user, FOO.objectid, FOO.LatestChange
from LOG
inner join
(
select objectid, max(changedate) as LatestChange
from LOG
group by objectid
) as FOO
on LOG.objectid = FOO.objectid and LOG.changedate = FOO.LatestChange

Answer 7

like martin smiths above,
simply just do a row number over partition and pick one of the rows that is most recent
like
SELECT Color,Date,User
FROM (
SELECT *,
ROW_NUMBER() OVER (PARTITION BY User ORDER BY [DATE]) AS ROW_NUMBER
FROM [tablename]
) AS ROWS
WHERE
ROW_NUMBER = 2