Question

So I am trying to run a query that will select, in this case, clients that haven’t had an appointment is X amount of time (2 weeks for instance). It boils down to “show me a list of clients that haven’t had an appointment in 2 weeks”. I’m attempting to solve this in php by doing something like:

$date = new DateTime;
$ago = new DateInterval('P2W');
$target = $date->sub($ago);
//query to select clients that aren't scheduled after the $target date
$clients = ...;

Two tables involved, appt_tbl, and clients_tbl. appt_tbl stores a client_id for each appointment record.

So essentially what I need is to select the “max” appointment for each client, and if it’s < my $target date, include them in the query results. I’ve tried various flavors of queries, queries with sub queries, but I’m stumbling on getting this query right.

My current attempt looks something like:

SELECT * 
FROM clients_tbl 
INNER JOIN
(
    SELECT client_id 
    FROM appt_tbl 
    WHERE MAX(appt_date_time) < '2012-07-22' 
    GROUP BY client_id
) appts ON appts.client_id = clients_tbl.client_id;

This should also include clients that have never been scheduled (IE won’t appear in the appt_tbl), but not clients who have an appointment booked in the next two weeks.

Answer 1

SELECT a.*
FROM clients_tbl a
LEFT JOIN appt_tbl b ON 
          a.client_id = b.client_id AND 
          b.appt_date_time >= CURDATE() - INTERVAL 2 WEEK 
WHERE b.client_id IS NULL

What this query does first (before the WHERE filtering) is select all clients whether or not they have a scheduled appointment greater than two weeks ago.

If the client does not have an appointment greater than two weeks ago, the values in the joined table will be NULL. We want all rows where the join conditions did not satisfy (i.e. values in joined table are null), which is done with WHERE b.client_id IS NULL.

This also includes into the result-set clients who do not have any corresponding appointments at all.

Clients who have appointments in the future are excluded.

There is also no need to construct a datetime string in PHP. You can simply do it straight in the query (although you may pass in the number of weeks ago as a parameter).