Preserve CASE WHEN output as new column in MySQL
ALTER TABLE dogs
ADD COLUMN Size VARCHAR(6) AS (CASE WHEN weight &lt; 15 THEN 'Small'
                                    WHEN weight &lt; 40 THEN 'Medium'
                                    ELSE 'Big' END) VIRTUAL

MySQL 8.0 Reference Manual  /  ...  /  CREATE TABLE and Generated Columns