问题
If I do a
dense_rank() over (order by colname),
I get the same rank for all rows with the same value in the column colname.
However, I want to limit the number of rows with the same rank to @maxrows so that when @maxrows rows have the same value in colname, a new rank is assigned to the next row even if the value of colname is still the same.
How can I achieve this?
回答1:
You can achieve this via using several ranking functions. We use ROW_NUMBER()
in the middle and another column to perform tie-breaking:
declare @maxRows int
set @maxRows = 5
; With InitialRanks as (
select DENSE_RANK() OVER (ORDER BY type) as rnk,* from sys.objects
), OrderedRanks as (
select (ROW_NUMBER() OVER (PARTITION BY rnk ORDER by object_id)-1)
/ @maxRows as rn,*
from InitialRanks
)
select DENSE_RANK() OVER (ORDER BY rnk,rn),* from OrderedRanks
Here I get only up to 5 columns with each (final) rank value. The ranking is based on type
but we use object_id
as a secondary column to work out the order in which rows are allowed a particular rank.
Turns out I'd over-complicated the above - there's no need for the first CTE and the first DENSE_RANK
since that's effectively acting as a proxy for the type
column in the ROW_NUMBER()
function - so why not just use the type
column directly and simplify things:
declare @maxRows int
set @maxRows = 5
; With OrderedRanks as (
select (ROW_NUMBER() OVER (PARTITION BY type ORDER by object_id)-1)
/ @maxRows as rn,*
from sys.objects
)
select DENSE_RANK() OVER (ORDER BY type,rn),* from OrderedRanks
来源:https://stackoverflow.com/questions/20656848/t-sql-dense-rank-with-a-maximum-number-of-rows-with-each-rank