Un ejemplo vale mas que mil palabras.
Código mysql:
Ver original+-------------+-------------+------+-------+
| id_paciente | nombre | hist | fecha |
+-------------+-------------+------+-------+
+-------------+-------------+------+-------+
+------+-------------+------------+
| hist | id_paciente | fecha |
+------+-------------+------------+
| 1 | 1 | 2009-06-30 |
| 2 | 3 | 2009-06-30 |
| 3 | 4 | 2009-06-30 |
| 4 | 2 | 2009-06-30 |
+------+-------------+------------+
->using(id_paciente
) set p.hist
=hc.hist
, p.fecha
=hc.fecha
; Query OK, 4 rows affected (0.00 sec)
+-------------+-------------+------+------------+
| id_paciente | nombre | hist | fecha |
+-------------+-------------+------+------------+
| 1 | daniel | 1 | 2009-06-30 |
| 2 | santiago | 4 | 2009-06-30 |
| 3 | juan manuel | 2 | 2009-06-30 |
| 4 | dondi | 3 | 2009-06-30 |
+-------------+-------------+------+------------+
Query OK, 0 rows affected (0.00 sec)
Cuentanos como te va.