MySQl数据约束练习

默认值约束

 1 CREATE TABLE USER(
 2      uid INT,
 3      uname   VARCHAR(10),
 4      address   VARCHAR(30) DEFAULT '山东省'
 5 )
 6 DROP TABLE USER;
 7 SELECT * FROM USER;
 8 INSERT INTO USER(uid,uname) VALUES(1,'张三');
 9 INSERT INTO USER VALUES(2,'李四','河南省');
10 INSERT INTO USER VALUES(3,'李四',NULL);

非空约束

1 CREATE TABLE USER(
2      uid INT NOT NULL,
3      uname   VARCHAR(10),
4      address   VARCHAR(30) DEFAULT '山东省'
5 )
6 INSERT INTO USER(uid,uname) VALUES(1,'张三');

唯一约束

1 CREATE TABLE USER(
2      uid INT UNIQUE,
3      uname   VARCHAR(10),
4      address   VARCHAR(30) DEFAULT '山东省'
5 )
6 INSERT INTO USER(uid,uname) VALUES(1,'张三');
7 INSERT INTO USER(uid,uname) VALUES(2,'张三');

主键约束(唯一+非空)

          1）被约束的表称为副表，约束别人的表称为主表，外键设置在副表上的！！！

          2）主表的参考字段通用为主键！

          3）添加数据： 先添加主表，再添加副表

          4）修改数据： 先修改副表，再修改主表

          5）删除数据： 先删除副表，再删除主表

 一个表内只能有一个主键。

1 CREATE TABLE USER(
2      uid INT  PRIMARY KEY,
3      uname   VARCHAR(10),
4      address   VARCHAR(30) DEFAULT '山东省'
5 )
6 SELECT * FROM USER;
7 INSERT INTO USER(uid,uname) VALUES(1,'张三');

 自增长约束AUTO_INCREMENT

1 CREATE TABLE USER(
2      uid INT  PRIMARY KEY AUTO_INCREMENT,
3      uname   VARCHAR(10),
4      address   VARCHAR(30) DEFAULT '山东省'
5 )
6 INSERT INTO USER(uname) VALUES('张三');

 零填充

1 CREATE TABLE USER(
2      uid INT(3) ZEROFILL  PRIMARY KEY AUTO_INCREMENT,
3      uname   VARCHAR(10),
4      address   VARCHAR(30) DEFAULT '山东省'
5 )
6 INSERT INTO USER(uname) VALUES('张三');

delete from 清空表不能删除约束

1 DELETE FROM USER;

TRUNCATE TABLE 清空表可以删除约束

1 TRUNCATE TABLE USER;

 外键约束(约束两种表的数据)

 1 CREATE  TABLE dept(
 2          deptid INT PRIMARY KEY AUTO_INCREMENT,
 3          deptname VARCHAR(10)
 4 )
 5 INSERT INTO dept(deptname) VALUES('软件开发部');
 6 INSERT INTO dept(deptname) VALUES('软件测试部');
 7 INSERT INTO dept(deptname) VALUES('软件运维部');
 8 SELECT * FROM dept;
 9 CREATE TABLE empt(
10        eid  INT PRIMARY KEY AUTO_INCREMENT,
11        ename  VARCHAR(5),
12        deptid INT,
13        CONSTRAINT dept_empt_fk FOREIGN KEY(deptid) REFERENCES dept(deptid)
14 )

新增副表

1 INSERT INTO empt(ename,deptid) VALUES('张三',1);
2 SELECT * FROM empt

修改主表

1 UPDATE dept SET deptname=2 WHERE deptid=1;
2 UPDATE empt SET deptid=3 WHERE eid=1;

删除主表

1 DELETE FROM dept WHERE deptid=2;