drop table if exists cTest;
create table cTest
(
patnum largeint not null,
mention smallint,
agent varchar(1),
primary key(patnum asc, mention asc)
);

--insert into cTest values(0,1,'');
--insert into cTest values(0,2,'');
--insert into cTest values(0,3,'');
--insert into cTest values(1,1,'');
--insert into cTest values(1,2,'');
insert into cTest values(1,3,'C');
--insert into cTest values(2,1,'');
insert into cTest values(2,2,'B');
--insert into cTest values(2,3,'');
--insert into cTest values(3,1,'');
insert into cTest values(3,2,'B');
insert into cTest values(3,3,'C');
insert into cTest values(4,1,'A');
--insert into cTest values(4,2,'');
--insert into cTest values(4,3,'');
insert into cTest values(5,1,'A');
--insert into cTest values(5,2,'');
insert into cTest values(5,3,'C');
insert into cTest values(6,1,'A');
insert into cTest values(6,2,'B');
--insert into cTest values(6,3,'');
insert into cTest values(7,1,'A');
insert into cTest values(7,2,'B');
insert into cTest values(7,3,'C');

select distinct patnum
into AandBandC
from cTest
where
 patnum in (select patnum from cTest where agent='A') and
 patnum in (select patnum from cTest where agent='B') and
 patnum in (select patnum from cTest where agent='C');

select patnum
into AandBandC_intersect
from cTest
where agent='A'
intersect
select patnum from cTest where agent='B'
intersect
select patnum from cTest where agent='C';