drop table visitor cascade constraints;
drop table event cascade constraints;
drop table uniqueticket cascade constraints;

create table visitor (
vid varchar2(1) primary key
);

create table event (
eid number primary key
);

create table uniqueticket (
vid varchar2(1) constraint fk1 references visitor(vid)
, eid number constraint fk2 references event(eid)
);

alter table uniqueticket
add constraint upk
primary key (vid, eid);

begin
for i in 1 .. 26
loop
insert into visitor values (chr(i+64));
insert into event values (i);
end loop;
end;
/

begin
for rec in (
select * from visitor cross join event
)
loop
insert into uniqueticket values (rec.vid, rec.eid);
end loop;
end;
/