谁帮我详细的解析一下这条SQL语句
if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[FK__aspnet_Us__Appli__4D6C0117]') and OBJECTPROPERTY(id, N'IsForeignKey') = 1)
ALTER TABLE [dbo].[aspnet_Users] DROP CONSTRAINT FK__aspnet_Us__Appli__4D6C0117
GO
if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[FK__aspnet_Us__Appli__4D6C0117]') and OBJECTPROPERTY(id, N'IsForeignKey') = 1)
ALTER TABLE [dbo].[aspnet_Users] DROP CONSTRAINT FK__aspnet_Us__Appli__4D6C0117
GO
删除该表的外键: FK__aspnet_Us__Appli__4D6C0117