if object_id('a','u')>o
alter table a drop column ...
else exec('create table a(...)')

------解决方案--------------------
IF OBJECT_ID('a') IS NOT NULL
  修改
else
  创建

------解决方案--------------------
SQL code
IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[TB]') 
AND type in (N'U')) --U 代表你查询的是表
DROP TABLE [TB]
GO

------解决方案--------------------
SQL code

IF EXISTS
(
SELECT 1 FROM SYSOBJECTS 
WHERE ID=OBJECT_ID(N'TABLENAME')
AND OBJECTPROPERTY(ID,N'ISTABLE')=1
)
ALTER TABLE
……
GO
ELSE
CREATE TABLE
……
GO