T-SQL :SQL Server 定义数据完整性 6大约束（三）

1.创建一客户张表

 IF OBJECT_ID('dbo.Employees', 'U') IS NOT NULL
   DROP TABLE dbo.Employees;

 CREATE TABLE dbo.Employees
 (
   empid     INT         NOT NULL,
   firstname ) NOT NULL,
   lastname  ) NOT NULL,
   hiredate  DATE        NOT NULL,
   mgrid     INT         NULL,
   ssn       ) NOT NULL,
   salary    MONEY       NOT NULL
 );

2.主键约束

-- Primary key
ALTER TABLE dbo.Employees
  ADD CONSTRAINT PK_Employees
  PRIMARY KEY(empid);
  

对于主键约束后台将创建一个唯一索引，以物理机制强制逻辑的唯一性约束

3.唯一约束

-- Unique
ALTER TABLE dbo.Employees
  ADD CONSTRAINT UNQ_Employees_ssn
  UNIQUE(ssn);

4.外键约束 创建一张订单表 插入外键

IF OBJECT_ID('dbo.Orders', 'U') IS NOT NULL
  DROP TABLE dbo.Orders;

CREATE TABLE dbo.Orders
(
  orderid   INT         NOT NULL,
  empid     INT         NOT NULL,
  custid    ) NOT NULL,
  orderts   DATETIME2   NOT NULL,
  qty       INT         NOT NULL,
  CONSTRAINT PK_Orders
    PRIMARY KEY(orderid)
);

-- Foreign keys
ALTER TABLE dbo.Orders
  ADD CONSTRAINT FK_Orders_Employees
  FOREIGN KEY(empid)
  REFERENCES dbo.Employees(empid);

5.CHECK约束

-- Check
ALTER TABLE dbo.Employees
  ADD CONSTRAINT CHK_Employees_salary
  CHECK(salary > 0.00);

6.默认约束

-- Default
ALTER TABLE dbo.Orders
  ADD CONSTRAINT DFT_Orders_orderts
  DEFAULT(SYSDATETIME()) FOR orderts;

-- Cleanup
DROP TABLE dbo.Orders, dbo.Employees;