Get distance between one date and another in SQL (ISO format)

Question

Let's say I have a table with two columns, where a user places a booking.

datefrom               dateto
-------------------------------------------
2009-05-23 00:00:00    2009-05-27 00:00:00

How would I use SQL to return how many days the user will be booking for? All I want is to return the number of days.

Answer 1

select days=datediff(dd,datefrom,dateto) from table

Answer 2

In Oracle:

select dateto - datefrom from table;

Answer 3

SQL-92:

SELECT (dateto - datefrom) DAY
  FROM MyTable;