MySQL选择内部联接（仅唯一）

Question

在我的应用中，我有分配给特定类别的车辆。 在下面的查询中，我试图选择其中某个区域内至少有一辆车辆的所有类别。

下面的代码运行良好，但是，如果类中有多个适合位置参数的车辆，则结果中将返回该类的多个实例。

如何构造该查询，以便无论类中有多少适合该参数的车辆返回每个类的一个实例？

$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

如果意图是仅返回类别表中的行，则可以用EXISTS (correlated subquery)替换JOIN

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
      )

---

如果还需要从vehicle表返回一行，则可以在内联视图中使用聚合，并在JOIN中使用。

 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：这假设id_class_cat在tbl_car_class_category是唯一的）

Answer 2

您尝试过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

Answer 3

为了获得每个类的单行，您可以使用（假的：min）聚合函数并分组

（在没有表模式的情况下，假设您使用col1，col2，col3而不是*）

$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");