第一個表是LocationTime表,第二個表是Student表。 我正在使用'ALTER'添加一個外鍵。但是我得到了下面的錯誤。爲什麼我會收到錯誤?MySQL:無法添加外鍵約束錯誤
無法添加外鍵約束錯誤
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| CourseN | varchar(3) | NO | PRI | NULL | |
| Quarter | varchar(11) | NO | PRI | NULL | |
| DayTime | varchar(7) | NO | PRI | NULL | |
| RoomN | varchar(3) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| studentName | varchar(15) | NO | PRI | NULL | |
| CourseN | varchar(3) | NO | PRI | NULL | |
| Quarter | varchar(11) | NO | PRI | NULL | |
+-------------+-------------+------+-----+---------+-------+
ALTER TABLE Student
ADD FOREIGN KEY (Quarter) REFERENCES LocationTime (Quarter)
ON UPDATE CASCADE ON DELETE SET NULL;