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.)