Iterated sub sampling against distinct values, union results - sql

Question

I made a SQL fiddle here
I have a table that has for each row: a category, an document id and a ranking.
The categories are ranked within themselves. For each category, I would like to select a sub sample. All the sub samples should be stacked together in a table.
The catch is that I would like to sub sample by iteratively fetching a halved row index among that category, e.g. if a given category has 32 items, then I would like to fetch rows 32, 16, 8, 4, 2, 1.
In my SQL fiddle I was able to do this for one particular category but I can't figure out how to:
a) do it for all categories in [Major Focus Area]
b) union the resulting subsamples into one table
Any hints or help is much appreciated! I am working in TSQL (MS SQL Server)
Sample data (MS Sql):
CREATE TABLE Rank_MajorAreas
([Rank] int, [Major Focus Area] varchar(17), [ID] int)
;
INSERT INTO Rank_MajorAreas
([Rank], [Major Focus Area], [ID])
VALUES
(1, 'Welfare', 71366),
(2, 'Welfare', 70415),
(3, 'Truck Driving', 70423),
(4, 'Peasant''s Office', 74566),
(5, 'Peasant''s Office', 71560),
(6, 'Nail Therapy', 77497),
(7, 'Truck Driving', 76193),
(8, 'Truck Driving', 79226),
(9, 'Truck Driving', 70222),
(10, 'Welfare', 77336),
(11, 'Truck Driving', 70823),
(12, 'Welfare', 77096),
(13, 'Welfare', 71335),
(14, 'Nail Therapy', 73551),
(15, 'Welfare', 72146),
(16, 'Truck Driving', 74023),
(17, 'Welfare', 71546),
(18, 'Nail Therapy', 74755),
(19, 'Peasant''s Office', 77834),
(20, 'Welfare', 75667),
(21, 'Peasant''s Office', 71342),
(22, 'Peasant''s Office', 77457),
(23, 'Peasant''s Office', 77923),
(24, 'Welfare', 76508),
(25, 'Welfare', 75714),
(26, 'Welfare', 73654),
(27, 'Welfare', 75753),
(28, 'Truck Driving', 71481),
(29, 'Truck Driving', 79424),
(30, 'Peasant''s Office', 76143),
(31, 'Truck Driving', 74076),
(32, 'Nail Therapy', 78714),
(33, 'Nail Therapy', 79924),
(34, 'Welfare', 71482),
(35, 'Welfare', 70050),
(36, 'Welfare', 76053),
(37, 'Nail Therapy', 79591),
(38, 'Peasant''s Office', 75197),
(39, 'Nail Therapy', 74104),
(40, 'Welfare', 72891),
(41, 'Truck Driving', 73621),
(42, 'Peasant''s Office', 71713),
(43, 'Welfare', 71979),
(44, 'Peasant''s Office', 71601),
(45, 'Peasant''s Office', 73928),
(46, 'Nail Therapy', 71759),
(47, 'Nail Therapy', 70379),
(48, 'Welfare', 71215),
(49, 'Truck Driving', 70908),
(50, 'Welfare', 71989)
;
Code thus far:
CREATE VIEW MFA AS
SELECT ROW_NUMBER() OVER(ORDER BY fa.[Rank] ASC) AS Row
,*
FROM Rank_MajorAreas AS fa
-- ideally we could make a view per Focus Area
WHERE fa.[Major Focus Area] = 'Welfare'
ORDER BY Row ASC
OFFSET 0 ROWS;
DECLARE #start int
SELECT #start = (SELECT COUNT(*) FROM MFA)
;WITH Sample( Row ) AS
(
Select #start as Row
UNION ALL
SELECT ROUND(Row/2, 0)
FROM Sample
WHERE Row > 0
)
SELECT * FROM MFA AS mfa
INNER JOIN Sample AS s on s.Row = mfa.Row
ORDER BY mfa.Row ASC
Desired Results, where each focus area is subsampled, the subsamples are returned all together as a single result
Row Rank Major Focus Area ID
1 1 Welfare 71366
2 2 Welfare 70415
4 12 Welfare 77096
9 24 Welfare 76508
19 50 Welfare 71989
...
1 6 Nail Therapy 77497
2 14 Nail Therapy 73551
4 32 Nail Therapy 78714
9 47 Nail Therapy 7037

Answer 1

You need to use PARTITION BY on Major Focus Area column in the OVER clause. Following is the modified TSQL
CREATE VIEW MFA AS
SELECT ROW_NUMBER() OVER(PARTITION BY fa.[Major Focus Area] ORDER BY fa.[Rank] ASC) AS Row
,*
FROM Rank_MajorAreas AS fa
-- ideally we could make a view per Focus Area
ORDER BY [Major Focus Area], Row ASC
OFFSET 0 ROWS;
DECLARE #start int
SELECT #start = (SELECT COUNT(*) FROM MFA)
;WITH Sample( Row, fa ) AS
(
Select COUNT(*) as Row, [Major Focus Area] as fa FROM MFA GROUP BY [Major Focus Area]
UNION ALL
SELECT ROUND(Row/2, 0), fa
FROM Sample
WHERE Row > 0
)
SELECT mfa.Row, mfa.Rank, mfa.[Major Focus Area] FROM MFA AS mfa
INNER JOIN Sample AS s on s.Row = mfa.Row and s.fa=mfa.[Major Focus Area]
ORDER BY [Major Focus Area], mfa.Row ASC
SQL fiddle