sql学习-约束

## 约束
CREATE TABLE USER(
	id int PRIMARY KEY auto_increment COMMENT '主键',
	NAME VARCHAR(10) not NULL UNIQUE COMMENT '姓名',
	age int CHECK ( age >0 && age <= 120 ) COMMENT '年龄',
	STATUS CHAR(1) DEFAULT '1' COMMENT '状态',
	gender CHAR(1) COMMENT '性别'
)COMMENT '用户表';

-- 插入数据
INSERT into user(name,age,STATUS,gender) VALUES ('Tom1',19,'1','男'),('Tom2',25,'0','男'),('Tom3',19,'1','男');

INSERT into user(name,age,STATUS,gender) VALUES ('Tom3',19,'1','男');
INSERT into user(name,age,STATUS,gender) VALUES (NULL,19,'1','男');

INSERT into user(name,age,STATUS,gender) VALUES ('Tom4',80,'1','男');

INSERT into user(name,age,STATUS,gender) VALUES ('Tom5',120,'1','男');

create table dept(
	id int auto_increment comment 'ID' primary key,
	name varchar(50) not null comment '部门名称'
)comment'部门表';

create table emp
(
id int auto_increment comment 'ID'  key,
name varchar(50) not null comment '姓名',
age int comment '年龄',
job varchar(20) comment '职位',
salary int comment '薪资',
entrydate date comment '入职时间',
managerid int comment '直质领导ID',
dept_id int comment '部门ID'
) comment '员工表1' ;
INSERT INTO dept(id,name) VALUES (1,'研发部'),(2,'市场部'),(3,'财务部'),(4,'销售部'),(5,'总经办');

INSERT INTO emp (id,name,age, job, salary, entrydate, managerid, dept_id) VALUES (1,'金庸',66,'总裁',20000,'2000-01-01',null,5), (2,'张无忌', 20, '项目经理', 12500, '2005-12-05',1,1),(3,'杨逍',33,'开发',8400,'2000-11-03',2,1),(4,'书一笑',48,'开发',11000,'2002-02-05',2,1),(5,'常遇春',43,'开发',10500,'2004-09-07',3,1),(6,'小昭',19,'程序员鼓励师',6600,'2004-10-12',2,1);

-- 添加外键
ALTER TABLE emp add CONSTRAINT fk_emp_dept_id FOREIGN KEY (dept_id) REFERENCES dept(id);

-- 删除外键
ALTER TABLE emp DROP FOREIGN KEY fk_emp_dept_id;

-- 外键的删除和更新行为
ALTER TABLE emp add CONSTRAINT fk_emp_dept_id FOREIGN KEY (dept_id) REFERENCES dept(id) on UPDATE CASCADE on DELETE CASCADE;


ALTER TABLE emp add CONSTRAINT fk_emp_dept_id FOREIGN KEY (dept_id) REFERENCES dept(id) on UPDATE set NULL on set null;