MySQL 8.0

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CREATE TABLE users (user_id INTEGER PRIMARY KEY, books VARCHAR(255));
INSERT INTO users (user_id, books) VALUES 
(1, 'book1, book2'), 
(2, 'book3, book4'); 

Records: 2  Duplicates: 0  Warnings: 0


SELECT * FROM users;


user_id
books


1
book1, book2

2
book3, book4


INSERT INTO users (user_id, books) VALUES 
(1, 'book1, book2'), 
(2, 'book3, book4') 
ON DUPLICATE KEY UPDATE 
books = CASE user_id
  WHEN 1 THEN 'book10, book20'
  WHEN 2 THEN 'book30, book40'
END;

Records: 2  Duplicates: 2  Warnings: 0


SELECT * FROM users;


user_id
books


1
book10, book20

2
book30, book40