Código sql:
Ver originalmysql> SELECT *FROM usuario_d;
+---------+---------------+
| user_id | user_password |
+---------+---------------+
| 1 | login1 |
| 2 | login2 |
| 3 | alejandro |
+---------+---------------+
3 ROWS IN SET (0.03 sec)
mysql> SELECT *FROM usuario_d WHERE user_id=1 AND user_id=3;
Empty SET (0.02 sec)
mysql> SELECT *FROM usuario_d WHERE user_id=1 OR user_id=3;
+---------+---------------+
| user_id | user_password |
+---------+---------------+
| 1 | login1 |
| 3 | alejandro |
+---------+---------------+
2 ROWS IN SET (0.00 sec)
mysql> SELECT *FROM usuario_d u1 INNER JOIN usuario_d u2 USING(user_id) WHERE u1.user_id=1 AND u2.user_id=3;
Empty SET (0.00 sec)
No es lo mismo?