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-- Hint: use Ctrl+Enter for SQL autocomplete CREATE TABLE Categories ( id int primar key auto_increment, name varchar(64), parent_id int -- references Categories ); CREATE TABLE Products ( id int primar key auto_increment, name varchar(64), category_id int, foreign key category_fk references Categories (id) ); INSERT INTO Categories VALUES (1, 'Root category', null), (2, 'Subcategory of Root category', 1); INSERT INTO Categories VALUES (3, 'Subcategory of not exists category', 6); SELECT * FROM Categories;