MySQL Select Inner Join (Only Unique) - php

Question

In my app, I have vehicles which are assigned to a specific class. In the query below I am trying to select all classes which have at least one vehicle in it that is in a certain area.
The code below works fine, however if there are multiple vehicles inside of the class that fit the location parameter, multiple instances of the class are returned in the results.
How can I structure this query so only one instance of every class is returned, regardless of how many vehicles inside of the class fit the paramater?
$get_veh = $pdo->prepare("SELECT * FROM tbl_car_class_category JOIN tbl_vehicles ON tbl_vehicles.veh_class_id = tbl_car_class_category.id_class_cat WHERE tbl_vehicles.veh_advanceLng between (:loc_long-:radius/cos(radians(:loc_lat))*69) and (:loc_long+:radius/cos(radians(:loc_lat))*69) and veh_advanceLat between (:loc_lat-(:radius/69)) and (:loc_lat+(:radius/69)) AND veh_status=:veh_status");
$get_veh->bindparam(":veh_status", $veh_status);
$get_veh->bindparam(":loc_lat", $loc_lat);
$get_veh->bindparam(":loc_long", $loc_long);
$get_veh->bindparam(":radius", $radius);
$get_veh->execute();

Answer 1

If the intent is return just rows from the category table, we could replace the JOIN with an EXISTS (correlated subquery)
For example:
SELECT c.*
FROM tbl_car_class_category c
WHERE EXISTS
( SELECT 1
FROM tbl_vehicles v
WHERE v.veh_class_id = c.id_class_cat
AND v.veh_advanceLng
BETWEEN (:loc_long-:radius/cos(radians(:loc_lat))*69)
AND (:loc_long+:radius/cos(radians(:loc_lat))*69)
AND v.veh_advanceLat
BETWEEN (:loc_lat-(:radius/69))
AND (:loc_lat+(:radius/69))
AND v.veh_status=:veh_status
)
If we also need to return a row from the vehicle table, we can use aggregation in an inline view, and a JOIN .
SELECT c.*
, g.*
FROM tbl_car_class_category c
JOIN ( SELECT v.veh_class_id
, MIN(v.id) AS min_id -- PK or unique identifier
FROM tbl_vehicles v
AND v.veh_advanceLng
BETWEEN (:loc_long-:radius/cos(radians(:loc_lat))*69)
AND (:loc_long+:radius/cos(radians(:loc_lat))*69)
AND v.veh_advanceLat
BETWEEN (:loc_lat-(:radius/69))
AND (:loc_lat+(:radius/69))
AND v.veh_status=:veh_status
GROUP BY v.veh_class_id
) m
ON m.veh_class_id = c.id_class_cat
JOIN tbl_vehicles g
ON g.id = m.min_id
AND g.veh_class_id = m.veh_class_id
ORDER BY ...
(NOTE: this assumes that id_class_cat is UNIQUE in tbl_car_class_category)

Answer 2

Have you tried LIMIT?
$get_veh = $pdo->prepare("SELECT * FROM tbl_car_class_category JOIN tbl_vehicles ON tbl_vehicles.veh_class_id = tbl_car_class_category.id_class_cat WHERE tbl_vehicles.veh_advanceLng between (:loc_long-:radius/cos(radians(:loc_lat))*69) and (:loc_long+:radius/cos(radians(:loc_lat))*69) and veh_advanceLat between (:loc_lat-(:radius/69)) and (:loc_lat+(:radius/69)) AND veh_status=:veh_status LIMIT 1");
http://www.mysqltutorial.org/mysql-limit.aspx
Or DISTINCT: https://dev.mysql.com/doc/refman/5.7/en/distinct-optimization.html

Answer 3

for get a single row for each class you can use a (fake eg: min) aggregation functio and group by
( do the absence of the table schema assuming that you have col1, col2, col3 instead of * )
$get_veh = $pdo->prepare("SELECT
tbl_car_class_category.id_class_cat
, min(col1)
, min(col2)
, min(col3)
FROM tbl_car_class_category
JOIN tbl_vehicles ON tbl_vehicles.veh_class_id = tbl_car_class_category.id_class_cat
WHERE tbl_vehicles.veh_advanceLng between (:loc_long-:radius/cos(radians(:loc_lat))*69)
and (:loc_long+:radius/cos(radians(:loc_lat))*69)
and veh_advanceLat between (:loc_lat-(:radius/69))
and (:loc_lat+(:radius/69)) AND veh_status=:veh_status
GROUP BY tbl_car_class_category.id_class_cat");