SELECT rows that are referenced less than x times in another table

Question

In MySQL I have two tables, my reservable "weekend":

id bigint(20) unsigned,
label varchar(64),
date_start date,
max_attendees smallint(5) unsigned

And my attendees:

id bigint(20) unsigned,
name varchar(64),
email varchar(255),
weekend bigint(20) unsigned

I want to select all weekends that have attendees less than their max_attendees. This includes weekends that have 0 attendees.

Note: I also need to ignore weekend with id "1" ;

Currently, this works fine with PHP (I'm using Wordpress for mysql access), like so:

$weekends = $wpdb->get_results("SELECT * FROM $weekends_table
                                   WHERE id <> 1", ARRAY_A);

$open_weekends = array();

foreach ($weekends as $weekend) {
    $id = $weekend['id'];
    $attendees = $wpdb->get_row("SELECT COUNT(id) as attendees
                                    FROM $attendees_table
                                    WHERE weekend = $id", ARRAY_A);

    if ( $attendees['attendees'] < $weekend['max_attendees'] ) {
        $weekend['attendees'] = $attendees['attendees'];
        $open_weekends[] = $weekend;
    }
}

Shouldn't I be able to do this in MySQL without the PHP? My knowledge of MySQL doesn't extend that far. Can you suggest a query?

Answer 1

use the HAVING clause

This is untested, so you may have to play with it, but here's the gist:

SELECT w.*, COUNT(a.name)
FROM weekend w
LEFT JOIN attendees a
  ON w.id = a.weekend
WHERE w.id <> 1
GROUP BY w.id
HAVING (COUNT(a.name) < w.max_attendees) OR (COUNT(a.name) IS NULL)

Answer 2

A very simple approach would be this:

SELECT COUNT($attendees_table.id) as attendees
       attendees_table.max_attendees as maximum
FROM weekends_table, attendees_table
WHERE attendees_table.weekend = weekends_table.id
GROUP BY weekends_table.id

You could use a JOIN ON attendees_table .

This should be possible as well:

SELECT COUNT(attendees_table.id) as attendees
       weekends_table.max_attendees as maximum
FROM weekends_table, attendees_table
WHERE attendees_table.weekend = weekends_table.id
GROUP BY weekends_table.id
HAVING attendees < maximum

This is all untested. I don't have your tables or data, but it might get you going?

Ah, it didn't get what you wanted. To include zero attendees you can use a subselect:

SELECT  weekends_table.id AS weekend_id
FROM weekends_table
WHERE weekends_table.max_attendees > (SELECT COUNT(*) 
                                      FROM attendees_table 
                                      WHERE attendees_table.weekend = weekends_table.id)

It should return weekend id's where there's at least room for one more attendee. Again, completely untested, but perhaps it works?