Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table Teacher (fam varchar(50))
insert into Teacher values ('2-2')
1 rows affected

insert into Teacher values ('2-2')
1 rows affected

SELECT max(cast(SUBSTRING(s.fam, 1, strpos(s.fam, '-')-1) as integer)) FROM Teacher s


max


2


--second problem is when you have text in strings
insert into Teacher values ('test-2')
1 rows affected

SELECT max(cast(SUBSTRING(s.fam, 1, strpos(s.fam, '-')-1) as integer)) FROM Teacher s
ERROR:  invalid input syntax for type integer: "test"