SQL how to implement if and else by checking column value - sql

Question

The table below contains customer reservations. Customers come and make one record in this table, and the last day this table will be updated its checkout_date field by putting that current time.
The Table
Now I need to extract all customers spending nights.
The Query
SELECT reservations.customerid, reservations.roomno, rooms.rate,
reservations.checkin_date, reservations.billed_nights, reservations.status,
DateDiff("d",reservations.checkin_date,Date())+Abs(DateDiff("s",#12/30/1899
14:30:0#,Time())>0) AS Due_nights FROM reservations, rooms WHERE
reservations.roomno=rooms.roomno;
What I need is, if customer has checkout status, due nights will be calculated checkin_date subtracting by checkout date instead current date, also if customer has checkout date no need to add extra absolute value from 14:30.
My current query view is below, also my computer time is 14:39 so it adds 1 to every query.

Answer 1

Since you want to calculate the Due nights upto the checkout date, and if they are still checked in use current date. I would suggest you to use an Immediate If.
The condition to check would be the status of the room. If it is checkout, then use the checkout_date, else use the Now(), something like.
SELECT
reservations.customerid,
reservations.roomno,
rooms.rate,
reservations.checkin_date,
reservations.billed_nights,
reservations.status,
DateDiff("d", checkin_date, IIF(status = 'checkout', checkout_date, Now())) As DueNights
FROM
reservations
INNER JOIN
rooms
ON reservations.roomno = rooms.roomno;
As you might have noticed, I used a JOIN. This is more efficient than merging the two tables with common identifier. Hope this helps !