create table comp_parts(id decimal primary key, Name Varchar)

create table comp_question(id decimal primary key,text varchar,id_part decimal references comp_parts(id))

create table comp_answers(id decimal primary key, text varchar, id_question decimal references comp_question(id))

insert comp_parts values ( 1 , 't')
insert comp_question values ( 1 , 'v',  1 )
insert comp_answers values ( 1 , 'a',  1 )

delete a from comp_answers a, comp_question q where a.id_question = q.id and q.id_part =  1 
delete comp_question where id_part =  1 
delete comp_parts where id =  1