数据库的五种约束 1.primary key 约束:设置主键约束 2.foreign key 约束：设置外键约束 3.unique 约束：设置唯一约束 4.default 约束：设置默认值约束 5.check 约束：通过逻辑表达式来判断数据的有效性，用来限制输入一系列或多列的值的范围
创建表、创建主键约束、唯一约束、外键约束、check约束、默认约束 create table employees ( empid int not null, firstname nvarchar(50) not null, lastname nvarchar(50) not null, hiredate date not null, mgrid int null, ssn nvarchar (50) not null, salary money not null, ) 添加主键约束 alter table employees add constraint PK_Employees primary key (empid) 添加唯一约束 alter table employees add constraint UNO_Employees unique (ssn) 添加外键约束 alter table employees add constraint PK_Employees_Employees foreign key (mgrid) references employees(empid) 添加check约束 alter table employees add constraint CHK_Employees check (salary>0.00) 添加默认约束 alter table employees add constraint DEF_Employees default (N’无’) for firstname
