Check

CREATE TABLE book
(
    book_id int,
    title text NOT NULL,
    isbn varchar(32) NOT NULL,
    publisher_id int,
    
    CONSTRAINT PK_book_book_id PRIMARY KEY (book_id),
    CONSTRAINT FK_book_publisher FOREIGN KEY (publisher_id) REFERENCES publisher(publisher_id)
);

ALTER TABLE book
ADD COLUMN price decimal CONSTRAINT CHK_book_price CHECK (price >= 0)