SQL Server: get median value - sql-server-2008

Question

How can i get the median value of each one of grouped values of a collumn from a select statement. I guess i use Row number but not sure how to go about it.
Note: if even number just take either middle no.
name size
joe 10
joe 11
joe 19
joe 20
joe 47
sally 3
sally 8
sally 57
john 1
john 3
I want to get Joe 19, Sally 8, John 3

Answer 1

Set up testing data:
CREATE TABLE #Foo (Name varchar(10), value int)
INSERT #Foo values
('joe', 10)
,('joe', 11)
,('joe', 19)
,('joe', 20)
,('joe', 47)
,('sally', 3)
,('sally', 8)
,('sally', 57)
,('john', 1)
,('john', 3)
First pass, get proper ordering:
SELECT Name, Value, row_number() over (partition by name order by value) ranking
from #Foo
Second pass, identify median item (If even number of items, the "first" item found is always returned)
;WITH cteRankings (Name, Value, Ranking)
as (select Name, Value, row_number() over (partition by name order by value) ranking
from #Foo)
SELECT Name, avg(Ranking) MedianItem
from cteRankings
group by Name
Final pass, get the details for the median item:
;WITH cteRankings (Name, Value, Ranking)
as (select Name, Value, row_number() over (partition by name order by value) Ranking
from #Foo)
SELECT cte.Name, cte.Value
from cteRankings cte
inner join (-- Median items
select Name, avg(Ranking) MedianItem
from cteRankings
group by Name) xx
on xx.Name = cte.Name
and xx.MedianItem = cte.Ranking
order by cte.Name
Done as a cte (common table expression), because a subquery was called for and needed to be referenced twice.