SQL Server 2019

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CREATE TABLE foo
(
  Id int,
  [Date] date,
  Operator varchar(10)
)

INSERT INTO foo (Id, [Date], Operator)
VALUES(1, '2021-01-01', 'op1'),
(2, '2021-01-02', 'op1'),
(3, '2021-02-01', 'op2'),
(4, '2021-02-02', 'op2'),
(5, '2021-02-03', 'op4'),
(6, '2021-01-03', 'op1');
6 rows affected

DELETE f
FROM foo f INNER JOIN 
(
  SELECT id, MONTH([Date]) m, ROW_NUMBER() OVER (PARTITION BY Operator, MONTH([Date]) ORDER BY id) rn
  FROM foo
) h ON f.id = h.id 
WHERE h.rn >1;
3 rows affected

SELECT *
FROM foo;


Id
Date
Operator


1
2021-01-01
op1

3
2021-02-01
op2

5
2021-02-03
op4