How to handle LEFT OUTER JOIN results efficiently with PHP / PDO - php

Question

Long time answer finder, first time asker here. I tried looking for an answer to this, but either it doesn't exists (doubtful) or I just can't seem to come up with the proper terminology to search with (most likely).
I'm using PHP / PDO to get data from a MariaDB database. The database has tables with a many-to-many relationship and a junction table between them like this:
people attendees events
+----+------+ +------+------+ +----+-------------------------+
| id | name | | p_id | e_id | | id | name |
+----+------+ +------+------+ +----+-------------------------+
| 1 | Bob | | 1 | 1 | | 1 | Company Christmas party |
| 2 | Anne | | 2 | 1 | | 2 | Christmas party cleanup |
| 3 | John | | 3 | 1 | | 3 | John's birthday party |
+----+------+ | 3 | 3 | +----+-------------------------+
+------+------+
I'm using a LEFT OUTER JOIN to select the events and their attendees:
SELECT events.id, events.name, people.id, people.name
FROM events
LEFT OUTER JOIN attendees
ON events.id = attendees.e_id
LEFT OUTER JOIN people
ON attendees.p_id = people.id
Now the "problem" is that the query returns multiples of the same event for each attendee as follows:
+----+-------------------------+------+------+
| id | name | id | name |
+----+-------------------------+------+------+
| 1 | Company Christmas party | 1 | Bob |
| 1 | Company Christmas party | 2 | Anne |
| 1 | Company Christmas party | 3 | John |
| 2 | Christmas party cleanup | NULL | NULL |
| 3 | John's birthday party | 3 | John |
+----+-------------------------+------+------+
Now I'm somewhat stuck with looping though the results. I want to create event objects from the results, but I want one event only as one object, adding attendees to it.
I could loop through my array of existing event objects and check if the id already exists when going through the resultset, but this seems a little inefficient. Is there a more elegant solution, possibly built into PDO?

Answer 1

I would do something like this:
Slightly modify your query (now we have different names for event name and person name):
SELECT events.id, events.name as event, people.id, people.name as person
FROM events
LEFT OUTER JOIN attendees
ON events.id = attendees.e_id
LEFT OUTER JOIN people
ON attendees.p_id = people.id
And parse results into array:
//$queryResult is the result returned by your query
while($row = $queryResult->fetch(PDO::FETCH_ASSOC)) {
$results[$row['event']][] = $row['person'];
}
In the end, you have an associative array that stores events and people.
print_r($results['Company Christmas party']) will return you
Array
(
[0] => Array
(
[0] => Bob
[1] => Anne
[2] => John
)
)
Regarding efficiency, I don't see anything inefficient here, linear complexity.