sql - selecting top N rows for each group in a table

Question

I am facing a very common issue regarding "Selecting top N rows for each group in a table".

Consider a table with id, name, hair_colour, score columns.

I want a resultset such that, for each hair colour, get me top 3 scorer names.

To solve this i got exactly what i need on Rick Osborne's blogpost "sql-getting-top-n-rows-for-a-grouped-query"

That solution doesn't work as expected when my scores are equal.

In above example the result as follow.

 id  name  hair  score  ranknum
---------------------------------
 12  Kit    Blonde  10  1
  9  Becca  Blonde  9  2
  8  Katie  Blonde  8  3
  3  Sarah  Brunette 10  1    
  4  Deborah Brunette 9  2 - ------- - - > if
  1  Kim  Brunette 8  3

Consider the row 4 Deborah Brunette 9 2. If this also has same score (10) same as Sarah, then ranknum will be 2,2,3 for "Brunette" type of hair.

What's the solution to this?

Answer 1

If you're using SQL Server 2005 or newer, you can use the ranking functions and a CTE to achieve this:

;WITH HairColors AS
(SELECT id, name, hair, score, 
        ROW_NUMBER() OVER(PARTITION BY hair ORDER BY score DESC) as 'RowNum'
)
SELECT id, name, hair, score
FROM HairColors
WHERE RowNum <= 3

This CTE will "partition" your data by the value of the hair column, and each partition is then order by score (descending) and gets a row number; the highest score for each partition is 1, then 2 etc.

So if you want to the TOP 3 of each group, select only those rows from the CTE that have a RowNum of 3 or less (1, 2, 3) --> there you go!