Sql_Server中怎樣推斷表中某列是否存在

/*推斷表AA中是否存在AA_ID這一列。假設不存在，則新增*/
IF NOT EXISTS (SELECT 1  FROM syscolumns INNER JOIN sysobjects ON sysobjects.id = syscolumns.id 
    WHERE syscolumns.name = 'AA_ID' AND sysobjects.name = 'AA')
ALTER TABLE AA ADD AA_ID VARCHAR(30) NULL   
GO