By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table t(id int null, id2 int null);
insert into t values (1, 1), (1, null), (3, 3), (null, null)
select count(*) as count
from t
| count |
|---|
| 4 |
select distinct *
from t
| id | id2 |
|---|---|
| 1 | 1 |
| 1 | null |
| 3 | 3 |
| null | null |
select count(*) as count
from (select distinct *
from t) x
| count |
|---|
| 4 |
select count(distinct id, id2) as count
from t
| count |
|---|
| 2 |