calcutate membership duration, until now or until end date - mysql

Question

I have a MySQL table with (among others) the following columns:
[name] [member_since_date] [member_until_date]
When somebody's membership ends, the [member_until_date] field is populated, otherwise it contains NULL.
I need a purely SQL based solution for the following:
I want to calculate how long someone is a member: when [member_until_date] is filled, I need it to calculate [member_until_date] - [member_since_date].
When somebody is still a member, the field [member_until_date] is NULL, so then I need it to calculate [NOW] - [member_since_date].
I hope I'm clear enough on this, and I hope somebody has an answer for me.

Answer 1

To get the difference between dates, in days, use DATEDIFF(). To take one value based on a condition, or another, use IF, though in this case I am using the similar IFNULL().
SELECT DATEDIFF(IFNULL(member_until_date, NOW()), member_since_date) AS days_member
FROM ...
IFNULL() says use the first argument, unless it's null, then use the second argument.
DATEDIFF() expects the larger date first in order to get a positive result.
COALESCE() provides similar functionality to IFNULL() and would be the ANSI SQL way of doing this.

Answer 2

Here's a solution using DATEDIFF().
SELECT name, DATEDIFF(IF(member_until_date,member_until_date,NOW()),
member_since_date) AS membership_duration
FROM members;
First, we check if member_until_date is null. If it is, we use NOW(), otherwise we use member_until_date.
IF(member_until_date,member_until_date,NOW())
Now we calculate the date difference between the above and the beginning of the membership, member_since_date, and return is as membership_duration.
DATEDIFF(IF(member_until_date,member_until_date,NOW()),
member_since_date) AS membership_duration