make foreign key in mariaDB code example

Example 1: mariadb add foreign key

alter table group_a 
add constraint fk_b_in_a
foreign key(`b_id`) 
    references group_b(`b_id`)
    on delete set null
    on update set null;

Example 2: alter table add foreign key mariadb example

alter table caixa add foreign key (idcaixafluxo) references caixafluxo (idcaixafluxo);