MySQL 8.0

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CREATE TABLE tablename (
  `Subs_ID` INTEGER,
  `Cust_ID` INTEGER,
  `C_Date` VARCHAR(10)
);

INSERT INTO tablename
  (`Subs_ID`, `Cust_ID`, `C_Date`)
VALUES
  ('123456', '2233', '2021-02-21'),
  ('123456', '2211', '2021-01-21'),
  ('123456', '3432', NULL),
  ('987654', '1122', '2021-02-21'),
  ('987654', '3322', '2021-01-21');
SELECT Subs_ID, Cust_ID, C_Date
FROM (
  SELECT *,
         ROW_NUMBER() OVER (
                        PARTITION BY Subs_ID 
                        ORDER BY CASE WHEN C_Date IS NULL THEN 1 ELSE 2 END, C_Date DESC 
                      ) rn
  FROM tablename
) t
WHERE rn = 1


Subs_ID
Cust_ID
C_Date


123456
3432
null

987654
1122
2021-02-21


SELECT Subs_ID, Cust_ID, C_Date
FROM (
  SELECT *,
         ROW_NUMBER() OVER (
                        PARTITION BY Subs_ID 
                        ORDER BY C_Date IS NULL DESC, C_Date DESC 
                      ) rn
  FROM tablename
) t
WHERE rn = 1


Subs_ID
Cust_ID
C_Date


123456
3432
null

987654
1122
2021-02-21