sql server - decrement a column from a table while decrement a column from another table -

i have 2 tables aprovizionari (which means supplies- of books) , facturi (which means invoices). tables have following structure:

create table aprovizionari (   id_aprovizionare  int     identity(1,1) primary key,    codcarte          char(3) foreign key references carti(codcarte),   codlibrarie       char(3) foreign key references librarii(codlibrarie),   dataaprovizionare date    default getdate(),    cantitate         int     default 1  -- ^^^^^^^ should decrement when cantitate facturi increment )  create table facturi (    codfactura  char(3) primary key,   codclient   char(4) foreign key references clienti(codclient),   codcarte    char(3) foreign key references carti(codcarte),   cantitate   int     default 1, -- ^^^^^^^ value here must decrement in aprovizionari.cantitate   codlibrarie char(3) foreign key references librarii(codlibrarie), ) 

aprovizionari (supply) stores cantitate (quantity) of each book in stock.

each record in facturi (invoices) stores purchase of quantity cantitate of particular book.

books can identified codcarte in both tables.

everytime invoice recorded in facturi, quantity of books invoiced should subtracted quantity of book available in aprovizionari. how can achieved?

if don't insert facturi in many different places should quite easy perform update on aprovizionari after insert, e.g.

insert facturi (codcarte, cantitate, ...) values (101, 2);  update aprovizionari set cantitate = cantitate - 2 codcarte = 101; 

otherwise trigger along lines of:

create trigger facturi_oninsert    on facturi    after insert  begin     update      set a.cantitate = a.cantitate - i.cantitate     aprovizionari     inner join inserted on i.codcarte = a.codcarte end