SQL Server 2017

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CREATE TABLE TestData
(
  UID INT,
  A INT,
  B INT
)
INSERT INTO TestData (UID, A, B)
VALUES (1, 2, 1)
, (2, 5, 1)
, (3, 4, 1)
, (4, 3, 2)
, (5, 8, 2)
, (6, 7, 2)
, (7, 4, 3)
, (8, 1, 3)
, (9, 6, 3)
9 rows affected

WITH MaxBs AS (
  SELECT B, MAX(A) AS A
  FROM TestData
  GROUP BY B
)

SELECT t.UID, t.A, t.B
FROM TestData t
INNER JOIN MaxBs b ON b.B = t.B and b.A = t.A
ORDER BY UID


UID
A
B


2
5
1

5
8
2

9
6
3


SELECT t.UID, t.A, t.B
FROM TestData t
INNER JOIN 
(
  SELECT MAX(A) as A, B
  FROM TestData
  GROUP BY B
) b ON t.A = b.A AND t.B = b.B


UID
A
B


9
6
3

5
8
2

2
5
1