MySQL外键 - 如何跨表强制执行一对一？

CREATE TABLE parent (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE child (
    id INT AUTO_INCREMENT PRIMARY KEY,
    parent_id INT,
    FOREIGN KEY (parent_id) REFERENCES parent(id)
);

INSERT INTO parent (name) VALUES ('Parent 1');
INSERT INTO child (parent_id, name) VALUES (1, 'Child 1');
INSERT INTO child (parent_id, name) VALUES (1, 'Child 2');
INSERT INTO child (parent_id, name) VALUES (1, 'Child 3');

ALTER TABLE child ADD FOREIGN KEY (parent_id) REFERENCES parent(id);

INSERT INTO child (parent_id, name) VALUES (1, 'Child 4');