MySQL 8.0

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    CREATE TABLE characters (
      id serial primary key,
      char_name TEXT NOT NULL
    );
    
    INSERT INTO characters (char_name) VALUES
     ('Harry'), ('Ron'), ('Hermione');

    SELECT * FROM characters;

Records: 3  Duplicates: 0  Warnings: 0

id
char_name

1
Harry

2
Ron

3
Hermione

    ALTER TABLE characters 
      ADD gender ENUM('m','f') AFTER char_name;

    SELECT * FROM characters;

Records: 0  Duplicates: 0  Warnings: 0

id
char_name
gender

1
Harry
null

2
Ron
null

3
Hermione
null

    INSERT INTO characters (id, char_name, gender) VALUES
      (1, '', 'm'), (2, '', 'm'), (3, '', 'f')
    ON DUPLICATE KEY UPDATE gender = VALUES(gender);

    SELECT * FROM characters;

Records: 3  Duplicates: 3  Warnings: 1

id
char_name
gender

1
Harry
m

2
Ron
m

3
Hermione
f