Cannot add foreign key constraint？

Question

mysql> CREATE TABLE province(

    -> id TINYINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,

    -> pname VARCHAR(20) NOT NULL

    -> );

Query OK, 0 rows affected (0.04 sec)

mysql> CREATE TABLE users(

    -> id TINYINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,

    -> username VARCHAR(20) NOT NULL,

    -> pid TINYINT,

    -> FOREIGN KEY (pid) REFERENCES province (id)

    -> );

ERROR 1215 (HY000): Cannot add foreign key constraint

Answer 1

刚发现了问题。id和pid没有同意符号位。