第一个表是LocationTime表,第二个表是Student表。 我正在使用'ALTER'添加一个外键。但是我得到了下面的错误。为什么我会收到错误?MySQL:无法添加外键约束错误
无法添加外键约束错误
+---------+-------------+------+-----+---------+-------+
| 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;