A表新增或删除记录时，B表的某一字段将相应的增加或减少

Insert:

Create Trigger tri_insert On Student
for Insert as
 Declare @cid int
 Select @cid=Cid From Inserted
 Update [Class] Set [Total]=[Total]+1 Where Cid=@cid

Go

Delete:

Create Trigger tri_insert On Student
for Delete as
 Declare @cid int
 Select @cid=Cid From Deleted
 Update [Class] Set [Total]=[Total]-1 Where Cid=@cid

Go