How can I select users who have not submitted records using php and mysql?

Question

I have columns in my database for date_created (format 2014-07-08 11:25:29) and username. I want to create a conditional statement which will display users who have not submitted a new record within the last week. I've never really queried my database for an action NOT taking place. How might I achieve this?

Answer 1

SELECT  username
FROM    records
GROUP BY
        username
HAVING  MAX(date_created) < NOW() - INTERVAL 1 WEEK

Note, however, that this query won't return users who haven't created a single record.

If you have another table holding all user records (including those who had no records in records ), use this query:

SELECT  u.username
FROM    users u
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    records r
        WHERE   r.username = u.username
                AND r.date_created >= NOW() - INTERVAL 1 WEEK
        )

In both cases, make sure you have an index on records (username, date_created) for the queries to work fast.