Partition by syntax - sql-server

Question

I have the following statement which works to get the most recent row of data for a particular DDI. What I now want to do is replace the single DDI in the where statement with a long list of them but still have only the most recent row for each. I'm pretty sure that I need to use OVER and PARTITION BY to get a separate window for each DDI but even reading the microsoft documentation and a more simplified tutorial I still can't get the syntax right. I suspect I just need a nudge in the right direction. Can anyone help?
https://learn.microsoft.com/en-us/sql/t-sql/queries/select-over-clause-transact-sql?view=sql-server-2017
http://www.sqltutorial.org/sql-window-functions/sql-partition-by/
SELECT TOP 1
[Start Time]
,[Agent Name]
,[Reference]
,[charged op. (sec)]
,[Type]
,[Activation ID] as [actid]
FROM [iPR].[dbo].[InboundCallsView]
Where [type] = 'Normal operator call'
AND [DDI] = #DDI
Order By [Start Time] Desc

Answer 1

Not sure how you plan on handling the multiple values for DDI but that may be an issue. The best approach would be to use a table valued parameter. If you pass in a delimited list you have to split the string too which is not a good way of handling this type of thing.
This query will return the most recent for every DDI.
SELECT
[Start Time]
, [Agent Name]
, [Reference]
, [charged op. (sec)]
, [Type]
, [actid]
from
(
SELECT
[Start Time]
, [Agent Name]
, [Reference]
, [charged op. (sec)]
, [Type]
, [actid]
, RowNum = ROW_NUMBER() over(partition by DDI order by [Start Time] desc)
FROM [iPR].[dbo].[InboundCallsView]
where [type] = 'Normal operator call'
--and [DDI] = #DDI
) x
where x.RowNum = 1

Answer 2

So let's assume a table with this data (notice how I cleaned up the column names to remove spaces, special characters, etc.):
+---+------------------+--------+------+----+------+---+
| 1 | 2019-03-28 08:00 | agent1 | foo1 | 60 | foo1 | 1 |
+---+------------------+--------+------+----+------+---+
| 1 | 2019-03-28 09:00 | agent2 | foo2 | 70 | foo2 | 2 |
| 2 | 2019-03-27 08:00 | agent3 | foo3 | 80 | foo3 | 3 |
| 2 | 2019-03-27 09:00 | agent4 | foo4 | 90 | foo4 | 4 |
+---+------------------+--------+------+----+------+---+
As you say, you can use a window function to get what you want. However, let me show you a method that doesn't require a window function first.
You want records where the StartTime is the max value for that DDI. You can obtain the max StartTime for each DDI with the following query:
SELECT
ddi,
max_start = MAX(StartTime)
FROM InboundCallsView
GROUP BY ddi
You can then join that query to your base table/view to get the records you want. Using an intermediate CTE, you can do the following:
WITH
ddiWithMaxStart AS
(
SELECT
ddi,
max_start = MAX(StartTime)
FROM InboundCallsView
GROUP BY ddi
)
SELECT InboundCallsView.*
FROM InboundCallsView
INNER JOIN ddiWithMaxStart ON
ddiWithMaxStart.ddi = InboundCallsView.ddi
AND ddiWithMaxStart.max_start = InboundCallsView.StartTime
Now, if you really want to use WINDOW functions, you can use ROW_NUMBER for a similar effect:
WITH
ddiWithRowNumber AS
(
SELECT
InboundCallsView.*,
rn = ROW_NUMBER() OVER
(
PARTITION BY ddi
ORDER BY ddi, StartTime DESC
)
FROM InboundCallsView
)
SELECT *
FROM ddiWithRowNumber
WHERE rn = 1
Notice that with this method, you don't need to join the base view/table to the intermediate CTE.
You can test out performance of each method to see which works best for you.