如何在SQL Server中创建判断表？

IF EXISTS ( 
 SELECT 1 FROM SYSOBJECTS T1 
 INNER JOIN SYSCOLUMNS T2 ON T1.ID=T2.ID 
 WHERE T1.NAME='A' AND T2.NAME='C' 
 ) 
 PRINT '存在' 
 ELSE 
 PRINT '不存在' 

IF COL_LENGTH('A', 'C') IS NOT NULL 
 PRINT N'存在' 
ELSE 
 PRINT N'不存在' 

select * from syscolumns where id=object_id('表名') and name='列名' 

select count(*) from sysobjects a,syscolumns b where a.id=b.id and b.name='flag1' and a.type='u' and a.name='T_Pro_ProductClass' 

if not exists (select * from sysobjects where id = object_id('mytab') 
and OBJECTPROPERTY(id, 'IsUserTable') = 1)
create table mytab
(
 id int,
 age int ,
 name varchar(max),
 primary key (id,age)
)
go

if not exists (select * from syscolumns where id=object_id('mytab') and name='columnname') alter table [mytab] add columnname nvarchar(max)