create table app_users
(
u_id Number,
user_name varchar2(50),
Flag varchar2(2),
create_by Number,
Mod_by Number,
constraint app_pk primary key(u_id));

create table payment
(
p_id varchar2(20),
p_date date,
f_date date,
created_by Number,
Mod_by Number,
constraint p_fk1 foreign key(mod_by) references app_users(u_id));

alter table payment add
constriant p_fk2 foreign key(created_by) references app_users(u_id));

alter table payment add
constriant p_fk2 foreign key(created_by) references app_users(u_id));