（三）创建、删除表

--判断是否存在当前table
IF(EXISTS(SELECT * FROM sys.objects WHERE name='classes'))
   DROP TABLE classes
GO 
--创建表classes
CREATE TABLE classes(
   id int primary key 	identity(1,2),   --主键：PRIMARY KEY;自动增长，起始值1，每次增长2的奇数列 
	 name varchar(22) not null,            --不为空：not null
	 createDate datetime default getDate() --默认为当前时间：default getDate()
)
GO


--创建表student
IF(EXISTS (SELECT * FROM sys.objects WHERE object_id=object_id('studentB')))
   DROP TABLE studentB 
GO 
CREATE TABLE studentB(
   id int identity(1,1)not null,
	 name varchar(20),
	 age int,
	 sex bit,
	 cid INT
)
GO