MySQL 5.6

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select version();


version()


5.6.45


create table tab( stud_group_id varchar(2), subject_id varchar(2));

insert into tab values
('1g','1s'),
('1g','2s'),
('1g','3s'),
('2g','1s'),
('2g','2s'),
('3g','1s'),
('3g','2s');
select subject_id         
  from tab
 group by subject_id
having count(stud_group_id) = ( select count(distinct stud_group_id) 
                              from tab )
                              


subject_id


1s

2s