在我的应用中,我有分配到特定课程的车辆。在下面的查询中,我试图选择其中至少有一辆车在某个区域内的所有类。
下面的代码工作正常,但是如果类中有多个适合location参数的工具,则会在结果中返回该类的多个实例。
如何构造此查询,以便只返回每个类的一个实例,无论该类内部有多少车辆符合参数?
$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();
答案 0 :(得分:2)
如果意图仅返回类别表中的行,我们可以将JOIN
替换为EXISTS (correlated subquery)
例如:
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
)
如果我们还需要从车辆表中返回一行,我们可以在内联视图和JOIN中使用聚合。
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 ...
(注意:这假定id_class_cat
中的tbl_car_class_category
是唯一的
答案 1 :(得分:0)
你试过LIMIT吗?
$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
或DISTINCT:https://dev.mysql.com/doc/refman/5.7/en/distinct-optimization.html
答案 2 :(得分:0)
为每个类获取一行,您可以使用(假的,例如:min)聚合函数和分组
(假设您有col1,col2,col3而不是*),缺少表模式
$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");