t-sql group by category and get top n values - tsql

Question

Imagine I have this table:
Month | Person | Value
----------------------
Jan | P1 | 1
Jan | P2 | 2
Jan | P3 | 3
Feb | P1 | 5
Feb | P2 | 4
Feb | P3 | 3
Feb | P4 | 2
...
How can I build a t-sql query to get the top 2 value rows and a third with the sum of others?
Something like this:
RESULT:
Month | Person | Value
----------------------
Jan | P3 | 3
Jan | P2 | 2
Jan | Others | 1 -(sum of the bottom value - in this case (Jan, P1, 1))
Feb | P1 | 5
Feb | P2 | 4
Feb | Others | 5 -(sum of the bottom values - in this case (Feb, P3, 3) and (Feb, P4, 2))
Thanks

Answer 1

In the assumption you are using SQL Server 2005 or higher, using a CTE would do the trick.
Attach a ROW_NUMBER to each row, starting with the highest value, resetting for each month.
SELECT the top 2 rows for each month from this query (rownumber <= 2)
UNION with the remaining rows (rownumber > 2)
SQL Statement
;WITH Months (Month, Person, Value) AS (
SELECT 'Jan', 'P1', 1 UNION ALL
SELECT 'Jan', 'P2', 2 UNION ALL
SELECT 'Jan', 'P3', 3 UNION ALL
SELECT 'Feb', 'P1', 5 UNION ALL
SELECT 'Feb', 'P2', 4 UNION ALL
SELECT 'Feb', 'P3', 3 UNION ALL
SELECT 'Feb', 'P4', 2
),
q AS (
SELECT Month
, Person
, Value
, RowNumber = ROW_NUMBER() OVER (PARTITION BY Month ORDER BY Value DESC)
FROM Months
)
SELECT Month
, Person
, Value
FROM (
SELECT Month
, Person
, Value
, RowNumber
FROM q
WHERE RowNumber <= 2
UNION ALL
SELECT Month
, Person = 'Others'
, SUM(Value)
, MAX(RowNumber)
FROM q
WHERE RowNumber > 2
GROUP BY
Month
) q
ORDER BY
Month DESC
, RowNumber
Kudo's go to Andriy for teaching me some new tricks.

Answer 2

;WITH atable (Month, Person, Value) AS (
SELECT 'Jan', 'P1', 1 UNION ALL
SELECT 'Jan', 'P2', 2 UNION ALL
SELECT 'Jan', 'P3', 3 UNION ALL
SELECT 'Feb', 'P1', 5 UNION ALL
SELECT 'Feb', 'P2', 4 UNION ALL
SELECT 'Feb', 'P3', 3 UNION ALL
SELECT 'Feb', 'P4', 2
),
numbered AS (
SELECT
Month, Person, Value,
rownum = ROW_NUMBER() OVER (PARTITION BY Month ORDER BY Value DESC)
FROM atable
),
grouped AS (
SELECT
Month, Person, Value,
Grp = CASE WHEN rownum < 3 THEN rownum ELSE 3 END
FROM numbered
)
SELECT
Month,
Person = CASE Grp WHEN 3 THEN 'Others' ELSE MAX(Person) END,
Value = SUM(Value)
FROM grouped
GROUP BY Month, Grp
ORDER BY Month DESC, Grp

Answer 3

WITH NTable AS
(
SELECT [Month],
Person,
Value,
ROW_NUMBER() OVER (PARTITION BY [Month] ORDER BY Value DESC)
AS Rownumber
FROM MyTable
)
SELECT t.[Month],
CASE Rownumber WHEN 1 THEN t.Person WHEN 2 THEN t.Person ELSE 'Others' END As Person,
SUM(t.Value) As [Sum]
FROM NTable t
GROUP BY t.[Month], CASE Rownumber WHEN 1 THEN t.Person WHEN 2 THEN t.Person ELSE 'Others' END
ORDER BY t.[Month]