#查看有没有索引：主键、外键、唯一键
SHOW INDEX FROM major;
SHOW INDEX FROM stuinfo;

#删除外键
ALTER TABLE stuinfo DROP FOREIGN KEY fk_stu_major;

#传统的方式添加外键
ALTER TABLE    stuinfo
ADD CONSTRAINT fk_stu_major FOREIGN KEY(majorid) REFERENCES major(id);

#查看表中有没有数据
SELECT * FROM major;

#插入数据
INSERT INTO major
VALUES (1,‘java‘),(2,‘H5‘),(3,‘大数据‘);

#查看表中有没有数据
SELECT * FROM stuinfo;

#插入数据
INSERT INTO stuinfo
SELECT 1,‘john1‘,‘女‘,NULL,NULL,1 UNION ALL
SELECT 2,‘john2‘,‘女‘,NULL,NULL,1 UNION ALL
SELECT 3,‘john3‘,‘女‘,NULL,NULL,2 UNION ALL
SELECT 4,‘john4‘,‘女‘,NULL,NULL,2 UNION ALL
SELECT 5,‘john5‘,‘女‘,NULL,NULL,1 UNION ALL
SELECT 6,‘john6‘,‘女‘,NULL,NULL,3 UNION ALL
SELECT 7,‘john7‘,‘女‘,NULL,NULL,3 UNION ALL
SELECT 8,‘john8‘,‘女‘,NULL,NULL,1 ;

#删除专业表中的三号专业
DELETE FROM major WHERE id=3;


#row: a foreign key 。出错有一个外键

#首先，把原来的外键删一遍
ALTER TABLE stuinfo DROP FOREIGN KEY fk_stu_major;

#然后，添加级联删除的外键
ALTER TABLE    stuinfo ADD CONSTRAINT fk_stu_major 
FOREIGN KEY(majorid) REFERENCES major(id) 
ON DELETE CASCADE;
#在原来添加外键的基础上，追加ON DELETE CASCADE

#最后，删除专业表的3号专业
DELETE FROM major WHERE id=3;
#删除完专业，学员信息也没了

#首先，把原来的外键删一遍
ALTER TABLE stuinfo DROP FOREIGN KEY fk_stu_major;

#然后，添加级联置空的外键
ALTER TABLE    stuinfo ADD CONSTRAINT fk_stu_major 
FOREIGN KEY(majorid) REFERENCES major(id) 
ON DELETE SET NULL;
#在原来添加外键的基础上，追加ON DELETE SET NULL

#最后，删除专业表的3号专业
DELETE FROM major WHERE id=2;
#删除完专业，学员信息专业的那一栏，引用为空