判断是否存在某列，并新增一列

语法：if exists(select 1 from syscolumns where name = '列名' and id = object_id('表名'))

例子：

IF NOT EXISTS (SELECT 1 FROM syscolumns WHERE id=OBJECT_ID('dbo.Shipping_Info') AND NAME='PaymentInfoID2')
BEGIN
    ALTER TABLE dbo.Shipping_Info ADD PaymentInfoID2 int NULL
END