sql - What should I use triggers or constraints? -

in sql developer created table

bill (id,nr,cost,days,total -all integers) 

and need create constraint (or trigger -what best) act

if nr > 10 total = total -5 (total = nr * cost * days ).   create or replace trigger discount  after insert or update on bill  update bill       set total = nr * cost * nr - 5; when (nr_slide > 10 ) 

can use update inside triggers?

constraint can prevent putting wrong value table. if use constraint, need calculate correct value total , insert table, otherwise no rows inserted. means need trigger.

create or replace trigger discount  after insert or update on bill  each row -- added here begin   if :new.nr > 10       :new.total = :new.nr * :new.cost * :new.days - 5;   end if; end; 

if educational project - no problem (of course, problem - quite bad example education). in applications real life use advise of ash burlaczenko.