Coalesce over Rows in MSSQL 2008, - sql

Question

I'm trying to determine the best approach here in MSSQL 2008.
Here is my sample data
TransDate Id Active
-------------------------
1/18 1pm 5 1
1/18 2pm 5 0
1/18 3pm 5 Null
1/18 4pm 5 1
1/18 5pm 5 0
1/18 6pm 5 Null
If grouped by Id and ordered by the TransDate, I want the last Non Null Value for the Active Column, and the MAX of TransDate
SELECT MAX(TransDate) AS TransDate,
Id,
--LASTNonNull(Active) AS Active
Here would be the results:
TransDate Id Active
---------------------
1/18 6pm 5 0
It would be like a Coalesce but over the rows, instead of two values/columns.
There would be many other columns that would also have this similiar method applied, so I really don't want to make a seperate join for each of the columns.
Any ideas?

Answer 1

I'd probably use a correlated sub query.
SELECT MAX(TransDate) AS TransDate,
Id,
(SELECT TOP (1) Active
FROM T t2
WHERE t2.Id = t1.Id
AND Active IS NOT NULL
ORDER BY TransDate DESC) AS Active
FROM T t1
GROUP BY Id
A way without
SELECT
Id,
MAX(TransDate) AS TransDate,
CAST(RIGHT(MAX(CONVERT(CHAR(23),TransDate,121) + CAST(Active AS CHAR(1))),1) AS BIT) AS Active,
/*You can probably figure out a more efficient thing to
compare than the above depending on your data. e.g.*/
CAST(MAX(DATEDIFF(SECOND,'19500101',TransDate) * CAST(10 AS BIGINT) + Active)%10 AS BIT) AS Active2
FROM T
GROUP BY Id
Or following the comments would cross apply work better for you?
WITH T (TransDate, Id, Active, SomeOtherColumn) AS
(
select GETDATE(), 5, 1, 'A' UNION ALL
select 1+GETDATE(), 5, 0, 'B' UNION ALL
select 2+GETDATE(), 5, null, 'C' UNION ALL
select 3+GETDATE(), 5, 1, 'D' UNION ALL
select 4+GETDATE(), 5, 0, 'E' UNION ALL
select 5+GETDATE(), 5, null,'F'
),
T1 AS
(
SELECT MAX(TransDate) AS TransDate,
Id
FROM T
GROUP BY Id
)
SELECT T1.TransDate,
Id,
CA.Active AS Active,
CA.SomeOtherColumn AS SomeOtherColumn
FROM T1
CROSS APPLY (SELECT TOP (1) Active, SomeOtherColumn
FROM T t2
WHERE t2.Id = T1.Id
AND Active IS NOT NULL
ORDER BY TransDate DESC) CA

Answer 2

This example should help, using analytical functions Max() OVER and Row_Number() OVER
create table tww( transdate datetime, id int, active bit)
insert tww select GETDATE(), 5, 1
insert tww select 1+GETDATE(), 5, 0
insert tww select 2+GETDATE(), 5, null
insert tww select 3+GETDATE(), 5, 1
insert tww select 4+GETDATE(), 5, 0
insert tww select 5+GETDATE(), 5, null
select maxDate as Transdate, id, Active
from (
select *,
max(transdate) over (partition by id) maxDate,
ROW_NUMBER() over (partition by id
order by case when active is not null then 0 else 1 end, transdate desc) rn
from tww
) x
where rn=1
Another option, quite expensive, would be doing it through XML. For educational purposes only
select
ID = n.c.value('#id', 'int'),
trandate = n.c.value('(data/transdate)[1]', 'datetime'),
active = n.c.value('(data/active)[1]', 'bit')
from
(select xml=convert(xml,
(select id [#id],
( select *
from tww t
where t.id=tww.id
order by transdate desc
for xml path('data'), type)
from tww
group by id
for xml path('node'), root('root'), elements)
)) x cross apply xml.nodes('root/node') n(c)
It works on the principle that the XML generated has each record as a child node of the ID. Null columns have been omitted, so the first column found using xpath (child/columnname) is the first non-null value similar to COALESCE.

Answer 3

You could use a subquery:
SELECT MAX(TransDate) AS TransDate
, Id
, (
SELECT TOP 1 t2.Active
FROM YourTable t2
WHERE t1.id = t2.id
and t2.Active is not null
ORDER BY
t2.TransDate desc
)
FROM YourTable t1

Answer 4

I created a temp table named #temp to test my solution, and here is what I came up with:
transdate id active
1/1/2011 12:00:00 AM 5 1
1/2/2011 12:00:00 AM 5 0
1/3/2011 12:00:00 AM 5 null
1/4/2011 12:00:00 AM 5 1
1/5/2011 12:00:00 AM 5 0
1/6/2011 12:00:00 AM 5 null
1/1/2011 12:00:00 AM 6 2
1/2/2011 12:00:00 AM 6 3
1/3/2011 12:00:00 AM 6 null
1/4/2011 12:00:00 AM 6 2
1/5/2011 12:00:00 AM 6 null
This query...
select max(a.transdate) as transdate, a.id, (
select top (1) b.active
from #temp b
where b.active is not null
and b.id = a.id
order by b.transdate desc
) as active
from #temp a
group by a.id
Returns these results.
transdate id active
1/6/2011 12:00:00 AM 5 0
1/5/2011 12:00:00 AM 6 2

Answer 5

Assuming a table named "test1", how about using ROW_NUMBER, OVER and PARTITION BY?
SELECT transdate, id, active FROM
(SELECT transdate, ROW_NUMBER() OVER(PARTITION BY id ORDER BY transdate desc) AS rownumber, id, active
FROM test1
WHERE active is not null) a
WHERE a.rownumber = 1