MySQL 8.0

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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

id	id2
1	1
1	null
3	3
null	null