mysql> select 1 from t;
+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
| 1 |
+---+
4 rows in set (0.00 sec)
mysql> select count(1) from t;
+----------+
| count(1) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from t;
+----------+
| count(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> select count(a) from t;
+----------+
| count(a) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)
mysql> explain extended SELECT count(*) FROM `employees`;
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | employees | index | NULL | PRIMARY | 4 | NULL | 299689 | 100.00 | Using index |
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.17 sec)
mysql> show warnings;
+-------+------+---------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select count(0) AS `count(*)` from `employees`.`employees` |
+-------+------+---------------------------------------------------------------------------+
1 row in set (0.17 sec)
mysql> explain extended SELECT count(1) FROM `employees`;
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | employees | index | NULL | PRIMARY | 4 | NULL | 299689 | 100.00 | Using index |
+----+-------------+-----------+-------+---------------+---------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+---------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select count(1) AS `count(1)` from `employees`.`employees` |
+-------+------+---------------------------------------------------------------------------+
1 row in set (0.00 sec)