T-SQL :SQL Server 定义数据完整性 6大约束（三）

1.创建一客户张表

 1 IF OBJECT_ID('dbo.Employees', 'U') IS NOT NULL
 2   DROP TABLE dbo.Employees;
 3 
 4 CREATE TABLE dbo.Employees
 5 (
 6   empid     INT         NOT NULL,
 7   firstname VARCHAR(30) NOT NULL,
 8   lastname  VARCHAR(30) NOT NULL,
 9   hiredate  DATE        NOT NULL,
10   mgrid     INT         NULL,
11   ssn       VARCHAR(20) NOT NULL,
12   salary    MONEY       NOT NULL
13 );

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    VARCHAR(10) 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;