In my database I have a Reservation table and it has three columns Initial Day, Last Day and the House Id.

I want to count the total days and omit those who are repeated, for example:

+-------------+------------+------------+
|             | Results    |            |
+-------------+------------+------------+
|   House Id  | InitialDay |  LastDay   |
+-------------+------------+------------+
|  1          | 2017-09-18 | 2017-09-20 |
|  1          | 2017-09-18 | 2017-09-22 |
| 19          | 2017-09-18 | 2017-09-22 |
| 20          | 2017-09-18 | 2017-09-22 |
+-------------+------------+------------+

If you noticed the House Id with the number 1 has two rows, and each row has dates but the first row is in the interval of dates of the second row. In total the number of days should be 5 because the first shouldn't be counted as those days already exist in the second.

The reason why this is happening is that each house has two rooms, and different persons can stay in that house on the same dates.

My question is: how can I omit those cases, and only count the real days the house was occupied?