How to get time difference (in hours) between 2 dates

I am trying to get the time difference between 2 users, I need the difference in hours.

I tried to use DATEDIFF function but it’s wrong.

Here is my code:

SELECT DATEDIFF(*,  
(SELECT max(u1.time_c)
FROM users u)
,
(SELECT max(u2.time_c)
FROM users u2) 

Answer

You must have a from clause in your select statement. Something like

Select date1 - date2 from dual

returns number of days between date1 and date2.

If you want number of hours:

Select (date1 - date2)*24 from dual;

(this is only for oracle)