create table a (m int, n int);
insert into a values (1, 2), (2, 3), (3, 4);
create table b (m int, n int);
insert into b values (1, 2), (1, 3), (3, 4);
create table c (m int, n int);
insert into c values (1, 3), (1, 3), (3, 4);
table a;
table b;
table c;
table a union all table b union all table c;