mysql> SELECT * FROM ALQUILER;
+------+---------------+------------+
| NIF | IDENTIFICADOR | FECHA |
+------+---------------+------------+
| 1 | uno | 2018-08-14 |
| 2 | dos | 2018-08-15 |
| 3 | tres | 2018-08-16 |
| 4 | cuatro | 2018-08-17 |
+------+---------------+------------+
4 rows in set (0.04 sec)
mysql> SELECT
-> ALQUILER.NIF,
-> ALQUILER.IDENTIFICADOR,
-> ALQUILER.FECHA,
-> DATEDIFF(CURDATE(),ALQUILER.FECHA)
-> FROM ALQUILER;
+------+---------------+------------+--------------------------------+
| NIF | IDENTIFICADOR | FECHA | DATEDIFF(CURDATE(),ALQUILER.FECHA) |
+------+---------------+------------+--------------------------------+
| 1 | uno | 2018-08-14 | 3 |
| 2 | dos | 2018-08-15 | 2 |
| 3 | tres | 2018-08-16 | 1 |
| 4 | cuatro | 2018-08-17 | 0 |
+------+---------------+------------+--------------------------------+
4 rows in set (0.00 sec)