drop table if exists emp;

CREATE table emp(
id INT PRIMARY KEY auto_increment , -- 员工id,自增长
ename VARCHAR(50) NOT NULL UNIQUE, -- 员工姓名，非空且唯一
joindate date NOT NULL, -- 入职日期，非空
salary DOUBLE(7,2) NOT NULL, -- 工资，非空
bonus DOUBLE(7,2) DEFAULT 0 -- 奖金，如果没有奖金默认为0
);

insert into emp(id,ename,joindate,salary,bonus)values(1,'张三','1990-11-11',8800,5000);

-- 演示主键约束：非空且唯一
insert into emp(id,ename,joindate,salary,bonus)values(null,'张三','1990-11-11',8800,5000);

insert into emp(id,ename,joindate,salary,bonus)values(1,'张三','1990-11-11',8800,5000);

insert into emp(id,ename,joindate,salary,bonus)values(2,'李四','1990-11-11',8800,5000);

-- 演示非空约束：
insert into emp(id,ename,joindate,salary,bonus)values(3,null,'1990-11-11',8800,5000);

-- 演示唯一约束：
insert into emp(id,ename,joindate,salary,bonus)values(4,'李四','1990-11-11',8800,5000);

-- 演示默认值约束：
insert into emp(id,ename,joindate,salary)values(3,'王五','1990-11-11',8800);

insert into emp(id,ename,joindate,salary,bonus)values(4,'赵六','1990-11-11',8800,null);

-- 演示自动增长：auto_increment ：当列是数字类型，并且他是唯一约束

insert into emp(ename,joindate,salary,bonus)values('赵六','1990-11-11',8800,null);
insert into emp(id,ename,joindate,salary,bonus)values(null,'赵六3','1990-11-11',8800,null);

SELECT * from emp;

还可以通过 alter table 表名 alter 列名 drop 约束 去删除约束