Why does using my SELF-JOIN gives me error?

Question

select 1.jmeno, 1.nadr
from zam as 1,
    zam as 2
where 1.nadr = 2.nadr
group by 1.jmeno, 1.nadr;

Answer 1

You need to change your alias name of the tables

select a1.jmeno, a1.nadr
from zam as a1 join
    zam as a2
on a1.nadr = a2.nadr
group by a1.jmeno, a1.nadr

Answer 2

use alias name by starting with letter and use explicit join not coma separated join

select t1.jmeno, t1.nadr
from zam as t1 join  zam as t2
on t1.nadr = t2.nadr

as there is no aggregation in your query so i removed group by instead that you could use distinct