create table a(
    a_id number primary key
);

--just child
create table b(
    b_id number primary key,
    a_id references a
);

--just child
create table c(
    c_id number primary key,
    a_id references a
);


-- child of the child

create table d(
    d_id number primary key,
    c_id references c
);

--child of two tables(one level)
create table e(
    e_id number primary key,
    b_id references b,
    c_id references c
);

--child of two tables(dependent)
create table f(
    e_id number primary key,
    c_id references c,
    d_id references d
);

--complex childs
create table g(
    g_id number primary key,
    e_id references e,
    f_id references f
);