Update trigger

I apologize in advance for the dirt in the code, but I don’t know how to do it in another way.

USE [ERP] GO /****** Object: Trigger [dbo].[Потребность] Script Date: 11.05.2016 19:59:12 ******/ SET ANSI_NULLS ON GO SET QUOTED_IDENTIFIER ON GO ALTER Trigger [dbo].[Потребность] ON [dbo].[Ведомость_материалов] FOR insert AS Declare @a int; Declare @b int; Declare @с int; Declare @v int; Declare @q int; select @с=Материал,@v=Колличество_на_складе from Ведомость_материалов select @a=sum(количество) from Заявка where Статус='В пути' AND Заявка.Наименование_детали=@с group by Наименование_детали select @b=количество from План where План.ID=@с select @q=Наименование_детали from Журнал_прихода update Ведомость_материалов set Чистая_потребность=@b-isnull(@a,0)-@v where Ведомость_материалов.Материал=@q 

his task is to calculate net profit by a simple formula

Net need = Quantity according to plan-quantity (only those that are still on the way) -quantity in stock

The problem is, when the Net need must be zero, then it takes on the value of the net need of another part.