mysql增添外键的4种体式格局
建立主表: 班级
CREATE TABLE class(cid INT PRIMARY KEY AUTO_INCREMENT, sname VARCHAR(15) NOT NULL) INSERT INTO class VALUES(NULL,'六年级一班') INSERT INTO class VALUES(NULL,'六年级二班')
第一种:在属性值背面直接增添
CREATE TABLE student(sid INT AUTO_INCREMENT, sname VARCHAR(10) NOT NULL, s_cid INT REFERENCES class(cid), PRIMARY KEY(sid) );
//增添数据举行测试
INSERT INTO student VALUES(NULL,'王大锤',2) INSERT INTO student VALUES(NULL,'胡汉三',3) //束缚不见效,缘由待查 INSERT INTO student VALUES(NULL,'卧槽',4) //不敢相信,在测试一次
这类体式格局,测试不知道为何,竟然增添进去了 WTF? 在网上百度无果,自学群无复兴 临时先放置在这儿
第二种:2.在sql语句末尾增添
CREATE TABLE student( sid INT AUTO_INCREMENT, sname VARCHAR(10), s_cid INT, PRIMARY KEY(sid), FOREIGN KEY (s_cid) REFERENCES class(cid) );
//插进去数据测试一下
INSERT INTO student VALUES(NULL,'王大锤',2) INSERT INTO student VALUES(NULL,'胡汉三',3) //束缚见效 插进去失利
外键束缚见效
第三种:fk的运用
CREATE TABLE student(sid INT AUTO_INCREMENT, sname VARCHAR(10) NOT NULL, s_cid INT , PRIMARY KEY(sid), CONSTRAINT fk_student_class FOREIGN KEY(s_cid) REFERENCES class(cid) );
INSERT INTO student VALUES(NULL,'王大锤',2) INSERT INTO student VALUES(NULL,'胡汉三',3) //束缚见效 插进去失利
第四种:在建表后增添外键
CREATE TABLE student(sid INT AUTO_INCREMENT, sname VARCHAR(10) NOT NULL, s_cid INT , PRIMARY KEY(sid) );
//增添外键束缚:
ALTER TABLE student ADD FOREIGN KEY (s_cid) REFERENCES class(cid);
//测试
INSERT INTO student VALUES(NULL,'王大锤',2) INSERT INTO student VALUES(NULL,'胡汉三',3) //束缚见效 插进去失利
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