How to effective get the max data in sub group and get the min data in among big groups? - sql

Question

Firstly, I hope to get the max date in each sub-groups.
Group A = action 1 & 2
Group B = action 3 & 4
actionName
action
actionBy
actiontime
999
1
Tom
2022-07-15 09:18:00
999
1
Tom
2022-07-15 15:21:00
999
2
Peter
2022-07-15 14:06:00
999
2
Peter
2022-07-15 14:08:00
999
3
Sally
2022-07-15 14:20:00
999
3
Mary
2022-07-15 14:22:00
999
4
Mary
2022-07-15 14:25:00
In this example:
The max time of group A is "1 | Tom | 2022-07-15 15:21:00 "
The max time of group B is " 4 | Mary | 2022-07-15 14:25:00 "
The final answer is "1 | Tom | 2022-07-15 14:25:00 ", which is the minimum data among groups.
I have a method how to get the max date in each group like the following code.
with cte1
as (select actionName,
actiontime,
actionBy,
row_number() over (partition by actionName order by actiontime desc) as rn
from actionDetails
where action in ( '1', '2' )
UNION
select actionName,
actiontime,
actionBy,
row_number() over (partition by actionName order by actiontime desc) as rn
from actionDetails
where action in ( '3', '4' )
)
select *
from cte1
where rn = 1
ActionName is not PK. It would get the max data in each group.
Then, I don't know how to use an effective way to get the minimum data between group A and group B. Would you give me some ideas?
I know one of the methods is self join again. However, I think that is not the best solution.

Answer 1

First of all, you can simplify your query by putting the action groups into the partition clause. Use a case expression to get one group for actions 1 and 2 and another for actions 3 and 4.
Then after getting the maximum dates per actionname and action group you want to get the minimum dates of these per actionname. This means you want a second CTE building up on the first one:
with max_per_group as
(
select top(1) with ties
actionname,
actiontime,
actionby
from actiondetails
where action in (1, 2, 3, 4)
order by row_number()
over (partition by actionname, case when action <= 2 then 1 else 2 end
order by actiontime desc)
)
, min_of_max as
(
select top(1) with ties
actionname,
actiontime,
actionby
from max_per_group
order by row_number() over (partition by actionname order by actiontime)
)
select actionname, actiontime, actionby
from min_of_max
order by actionname;
As you see, instead of computing a row number and then have to limit rows based on that in the next query, I limit the rows right away by putting the row numbering into the ORDER BY clause and applying TOP(1) WITH TIES to get all rows numbered 1. I like this a tad better, because the CTE already produces the rows that I want to work with rather than only marking them in a bigger data set. But that's personal preference I guess.
Discaimer:
In my query I assume that the column action is numeric. If the column is a string instead, because it can hold values that are not numbers, then work with strings:
where action in ('1', '2', '3', '4')
partition by actionname, case when action in ('1', '2') then 1 else 2 end
If on the other hand the column is a string, but there are only numbers in that column, fix your table instead.