Selecting rows based on date and other tables

Question

I have three tables:

computers, which has columns for id, serial number, make, model, etc
users, which has columns for id, real name
computer_usage, which has columns id, computer_id, user_id, start_date, end_date

The database records all computers that we have, our users, and who used what computer when.

I want to run a query that selects all computers that do not have a current user.

This would be:

There is a computer_id that doesn't exist at all within computer_usage or There is a computer_id that does exist within computer_usage that has an end_date of

What is the best way of selecting those records?

Answer 1

Try this:

SELECT * 
FROM computers 
WHERE id NOT IN 
    (SELECT computer_id 
     FROM computer_usage
     WHERE end_date is NULL)

The subquery shows you all computers that are currently in use (a record exists, so we assume it has a start_date, but no end_date has been assigned).

I'm assuming here that the end_date doesn't get populated until the "usage" is complete. If that isn't correct (if the end dates are pre-assigned, for example), then just do:

SELECT * 
FROM computers 
WHERE id NOT IN 
    (SELECT computer_id 
     FROM computer_usage
     WHERE start_date < getDate()
     AND end_date > getDate())

The subquery still shows all computers that are currently "on assignment". Just find all computers not in that group. (Note that I've used "getDate", which is a MS SQL function. I think the mysql equivalent is something having to do with TIMESTAMP.)