SQL server 触发器问题

/**********设A B C三表在同一数据库F，触发器名updatea
USE [F]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER  TRIGGER [dbo].[updatea]
   ON [dbo].[C]
   FOR INSERT
AS
BEGIN
   DECLARE @valuebb int,@valuecc int,@valueid int;
   SELECT @valueid=id FROM inserted
   SET @valuebb=(SELECT SUM(b) FROM B WHERE id=@valueid);
   SET @valuecc=(SELECT SUM(c) FROM C WHERE id=@valueid);
   IF @valuebb=@valuecc
   BEGIN
      UPDATE dbo.A SET a=@valuebb-@valuecc WHERE id=@valueid
   END
END