老師你2-13這一集的01:10~01:25時段應該剛錯了，UNIQUE KEY 約束的字段其實可以有多個空值，你說成只能有一個空值了
>show columns from user4;
+----------+---------------------+------+-----+---------+----------------+
| Field??? | Type??????????????? | Null | Key | Default | Extra????????? |
+----------+---------------------+------+-----+---------+----------------+
| id?????? | smallint(6)???????? | NO?? | PRI | NULL??? | auto_increment |
| username | varchar(20)???????? | YES? | UNI | NULL??? |??????????????? |
| age????? | tinyint(3) unsigned | YES? |???? | NULL??? |??????????????? |
+----------+---------------------+------+-----+---------+----------------+
>select * from user4;
+----+----------+------+
| id | username | age? |
+----+----------+------+
|? 1 | NULL???? |?? 23 |
|? 2 | tom????? |?? 23 |
|? 3 | NULL???? |?? 24 |
+----+----------+------+