Perform ranking depend on category - sql

Question

I Have a table looks like this:
RowNum category Rank4A Rank4B
-------------------------------------------
1 A
2 A
3 B
5 A
6 B
9 B
My requirement is based on the RowNum order, Make two new ranking columns depend on category. Rank4A works like the DENSERANK() by category = A, but if the row is for category B, it derives the latest appeared rank for category A order by RowNum. Rank4B have similar logic, but it orders by RowNum in DESC order. So the result would like this (W means this cell I don't care its value):
RowNum category Rank4A Rank4B
-------------------------------------------
1 A 1 W
2 A 2 W
3 B 2 3
5 A 3 2
6 B W 2
9 B W 1
One more additional requirement is that CROSS APPLY or CURSOR is not allowed due to dataset being large. Any neat solutions?
Edit: Also no CTE (due to MAX 32767 limit)

Answer 1

You can use the following query:
SELECT RowNum, category,
SUM(CASE
WHEN category = 'A' THEN 1
ELSE 0
END) OVER (ORDER BY RowNum) AS Rank4A,
SUM(CASE
WHEN category = 'B' THEN 1
ELSE 0
END) OVER (ORDER BY RowNum DESC) AS Rank4B
FROM mytable
ORDER BY RowNum

Answer 2

Giorgos Betsos' answer is better, please read it first.
Try this out. I believe each CTE is clear enough to show the steps.
IF OBJECT_ID('tempdb..#Data') IS NOT NULL
DROP TABLE #Data
CREATE TABLE #Data (
RowNum INT,
Category CHAR(1))
INSERT INTO #Data (
RowNum,
Category)
VALUES
(1, 'A'),
(2, 'A'),
(3, 'B'),
(5, 'A'),
(6, 'B'),
(9, 'B')
;WITH AscendentDenseRanking AS
(
SELECT
D.RowNum,
D.Category,
AscendentDenseRanking = DENSE_RANK() OVER (ORDER BY D.Rownum ASC)
FROM
#Data AS D
WHERE
D.Category = 'A'
),
LaggedRankingA AS
(
SELECT
D.RowNum,
AscendentDenseRankingA = MAX(A.AscendentDenseRanking)
FROM
#Data AS D
INNER JOIN AscendentDenseRanking AS A ON D.RowNum > A.RowNum
WHERE
D.Category = 'B'
GROUP BY
D.RowNum
),
DescendantDenseRanking AS
(
SELECT
D.RowNum,
D.Category,
DescendantDenseRanking = DENSE_RANK() OVER (ORDER BY D.Rownum DESC)
FROM
#Data AS D
WHERE
D.Category = 'B'
),
LaggedRankingB AS
(
SELECT
D.RowNum,
AscendentDenseRankingB = MAX(A.DescendantDenseRanking)
FROM
#Data AS D
INNER JOIN DescendantDenseRanking AS A ON D.RowNum < A.RowNum
WHERE
D.Category = 'A'
GROUP BY
D.RowNum
)
SELECT
D.RowNum,
D.Category,
Rank4A = ISNULL(RA.AscendentDenseRanking, LA.AscendentDenseRankingA),
Rank4B = ISNULL(RB.DescendantDenseRanking, LB.AscendentDenseRankingB)
FROM
#Data AS D
LEFT JOIN AscendentDenseRanking AS RA ON D.RowNum = RA.RowNum
LEFT JOIN LaggedRankingA AS LA ON D.RowNum = LA.RowNum
LEFT JOIN DescendantDenseRanking AS RB ON D.RowNum = RB.RowNum
LEFT JOIN LaggedRankingB AS LB ON D.RowNum = LB.RowNum
/*
Results:
RowNum Category Rank4A Rank4B
----------- -------- -------------------- --------------------
1 A 1 3
2 A 2 3
3 B 2 3
5 A 3 2
6 B 3 2
9 B 3 1
*/
This isn't a recursive CTE, so the limit 32k doesn't apply.