DROP TABLE IF EXISTS comments;
DROP TABLE IF EXISTS posts;

CREATE TABLE posts (
id INT NOT NULL AUTO_INCREMENT,
message VARCHAR(140),
PRIMARY KEY (id)
);

CREATE TABLE comments (
id INT NOT NULL AUTO_INCREMENT,
post_id INT,
comment VARCHAR(140),
PRIMARY KEY (id),
FOREIGN KEY (post_id) REFERENCES posts(id)
ON DELETE CASCADE
ON UPDATE CASCADE
);

INSERT INTO posts (message) VALUES
('post-1'),
('post-2'),
('post-3'),
('new post');

INSERT INTO comments (post_id, comment) VALUES
(1, 'comment-1-1'),
(2, 'comment-1-2');

INSERT INTO comments (post_id, comment) VALUES
(LAST_INSERT_ID(), 'new comment');

SELECT * FROM posts;
SELECT * FROM comments;

このようなコードの時に、LAST_INSERT_ID()で取得したpost_idが1になるのですが、この１はどこからきているのですか？