MySQL 8.0

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CREATE TABLE tablename (`ID` VARCHAR(6));

INSERT INTO tablename (`ID`)
VALUES
  ('000001'),
  ('011111');
SELECT ID, 
       CONCAT(REPEAT('X', LENGTH(ID) - LENGTH(ID + 0)), ID) `Return`
FROM tablename


ID
Return


000001
XXXXX000001

011111
X011111


SELECT ID, 
       CONCAT(REPEAT('X', LENGTH(ID) - LENGTH(TRIM(LEADING '0' FROM ID))), ID) `Return`
FROM tablename


ID
Return


000001
XXXXX000001

011111
X011111