SQL Server 2019

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CREATE TABLE Clients
(
    ClientID  INT NOT null,
	ClientName VARCHAR(35) NOT null
)

CREATE TABLE ClientOperations
(
    ClientID INT NOT null
  , OperationPerformed VARCHAR(35) NOT null
  , OperationDateTime DATETIME NOT null
);

INSERT INTO Clients
(
    ClientID
  , ClientName
)
VALUES (1, 'Client 1 Name')
     , (2, 'Client 2 Name')
     , (3, 'Client 3 Name')
     , (4, 'Client 4 Name')

INSERT INTO ClientOperations
(
    ClientID
  , OperationPerformed
  , OperationDateTime
)
VALUES (1, 'Operation 11', '2022-06-24 12:04:53')
     , (1, 'Operation 12', '2022-07-30 09:21:28')
     , (2, 'Operation 21', '2022-05-01 10:32:57')
     , (2, 'Operation 22', '2022-05-29 03:28:09')
     , (2, 'Operation 32', '2022-06-15 04:14:26')
     , (3, 'Operation 31', '2022-04-29 11:49:36')

SELECT * FROM Clients

SELECT * FROM ClientOperations

SELECT
          c.ClientID
        , c.ClientName
        , co.OperationPerformed
        , co.OperationDateTime
FROM      Clients c
LEFT JOIN (
              SELECT
                    *
                  , ROW_NUMBER () OVER (PARTITION BY ClientID ORDER BY OperationDateTime DESC) AS rn
              FROM  ClientOperations
              WHERE CONVERT (DATE, OperationDateTime) >= CONVERT (DATE, DATEADD (MONTH, -3, GETDATE ()))
          )        co ON c.ClientID = co.ClientID
                     AND co.rn      = 1;

ClientID	ClientName
1	Client 1 Name
2	Client 2 Name
3	Client 3 Name
4	Client 4 Name

ClientID	OperationPerformed	OperationDateTime
1	Operation 11	2022-06-24 12:04:53.000
1	Operation 12	2022-07-30 09:21:28.000
2	Operation 21	2022-05-01 10:32:57.000
2	Operation 22	2022-05-29 03:28:09.000
2	Operation 32	2022-06-15 04:14:26.000
3	Operation 31	2022-04-29 11:49:36.000

ClientID	ClientName	OperationPerformed	OperationDateTime
1	Client 1 Name	Operation 12	2022-07-30 09:21:28.000
2	Client 2 Name	Operation 32	2022-06-15 04:14:26.000
3	Client 3 Name	null	null
4	Client 4 Name	null	null