Display Alphabetical order a list of name and surname - php

Question

I need to display in alphabetical order a list of name and surname (order by surname).
The problem is I can't do a ORDER BY in SQL query because I retrieve my users ID in one table and retrieve the informations in another table.
My PHP code:
$sql = "select id FROM $table_name";
$result = $wpdb->get_results($sql);
foreach ($result as $record) {
$id = $record->id;
$sql2 = "select field_name, field_val FROM $table_name2 where sub_id = $id";
$result2 = $wpdb->get_results($sql2);
foreach ($result2 as $record2) {
if($record2->field_name == "Nom :") {
$surname = ucfirst(stripslashes($record2->field_val));
}
if($record2->field_name == "Prénom :") {
$name = ucfirst(stripslashes($record2->field_val));
}
}
echo $name . " " . $surname . "<br/>";
}
Here the architecture of the second table:
f_id sub_id field_name field_val
127 19 Prénom : Philippe
128 19 Nom : Nailloux
129 20 Prénom : John
130 20 Nom : Drumond
Have you an idea how I can display my list ordered by surname alphabetically?
Thanks.

Answer 1

You can do the trick by using this SQL query :
SELECT t1.id AS user_id,
t2.field_val AS surname,
t3.field_val AS name
FROM $table_name t1
JOIN $table_name2 AS t2
ON ( t2.sub_id = t1.id
AND t2.field_name = 'Nom :' )
JOIN $table_name2 AS t3
ON ( t3.sub_id = t1.id
AND t3.field_name = 'Prénom :' )
ORDER BY t2.field_val
The query will return all the infos needed (user_id, surname and name) ordered by surname.

Answer 2

Use subqueries!
In your example, let's say you have 100 users.
In first query you get your users ids, then for each user you query database for one's data. That's 101 queries for a simple operation! What if you have 10 visits per seconds? That's more than 1000 queries.
There are many strategies (subquery, join, two queries with in () in second one) but in this case consider that:
SELECT
field_name, field_val
FROM
$table_name2
WHERE
sub_id IN(
SELECT
id
FROM
$table_name
) as sq1
ORDER BY field_val ASC;
Also consider using PHP PDO, or at least proper escaping of data you put in queries.