create table mytable (c1 string,c2 string);
insert into mytable values ('A','A'),('B',null),('C',null);

select count(distinct c1) as count_distinct from mytable;

+----------------+
| count_distinct |
+----------------+
|              3 |
+----------------+

select count(distinct concat(c1,c2)) as count_distinct from mytable;

+----------------+
| count_distinct |
+----------------+
|              1 |
+----------------+

select c1,c2,concat(c1,c2) as concat_c1_c2 from mytable;

+----+------+--------------+
| c1 |  c2  | concat_c1_c2 |
+----+------+--------------+
| A  | A    | AA           |
| B  | NULL | NULL         |
| C  | NULL | NULL         |
+----+------+--------------+

select count(distinct c1,c2) as count_distinct from mytable;

+----------------+
| count_distinct |
+----------------+
|              1 |
+----------------+

select distinct c1,c2 from mytable;

+----+------+
| c1 |  c2  |
+----+------+
| B  | NULL |
| A  | A    |
| C  | NULL |
+----+------+

select count(distinct struct(c1,c2)) as count_distinct from mytable;

+----------------+
| count_distinct |
+----------------+
|              3 |
+----------------+