mysql> CREATE TABLE nulls (a INT(11), b INT(11) NOT NULL);

mysql> INSERT INTO nulls VALUES(1, 2), (NULL, 2);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> SELECT COUNT(*), COUNT(a), COUNT(b) FROM nulls;
+----------+----------+----------+
| COUNT(*) | COUNT(a) | COUNT(b) |
+----------+----------+----------+
|        2 |        1 |        2 |
+----------+----------+----------+
1 row in set (0.02 sec)

mysql> SELECT * FROM nulls;
+------+---+
| a    | b |
+------+---+
|    1 | 2 |
| NULL | 2 |
+------+---+
2 rows in set (0.01 sec)

mysql> EXPLAIN SELECT COUNT(*) FROM nulls;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT COUNT(a) FROM nulls;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | nulls | ALL  | NULL          | NULL | NULL    | NULL | 2    |       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> EXPLAIN SELECT COUNT(b) FROM nulls;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)