MySQL 游标循环查找最小值

Question

这是我要完成的任务：

找到给定位置最近的可用车辆。

我有一个vehicle 的表和一个location 的表，如下所示：

CREATE TABLE location
  (location_id numeric(8,0) UNIQUE NOT NULL,
   address varchar(100), 
   latitude float,
   longitude float,
   PRIMARY KEY(location_id)
 );

CREATE TABLE vehicle
  (license_plate char(6) UNIQUE NOT NULL,
   make varchar(30) NOT NULL,
   model varchar(30) NOT NULL,
   year numeric(4,0) NOT NULL CHECK(year>1990),
   state char(2) NOT NULL, 
   capacity int NOT NULL,
   last_location numeric(8,0) DEFAULT NULL,
   FOREIGN KEY(last_location) REFERENCES location(location_id) ON DELETE 
   CASCADE ON UPDATE CASCADE,
   PRIMARY KEY(license_plate)
 );

我编写了一个查询，它调用一个函数来循环遍历vehicle 表，计算与给定位置的距离并返回具有最小距离的汽车的license_plate。

SELECT @locationA := 11111111;
SET @loc_lat = (SELECT latitude FROM location WHERE location_id = 
@locationA);
SET @loc_long = (SELECT longitude FROM location WHERE location_id = 
@locationA);
SELECT license_plate, make, model FROM vehicle
WHERE license_plate = find_car(@loc_lat, @loc_long); 

DELIMITER $$
CREATE FUNCTION find_car(loc_lat float, loc_long float) RETURNS char  
BEGIN
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
    DECLARE CUR_DIST float DEFAULT 1000000000; 
    DECLARE car_lat NUMERIC; 
    DECLARE car_long NUMERIC; 
    DECLARE dist float; 
    DECLARE closest_car char(6); 
    DECLARE car_temp char(6);
    DECLARE loc_temp numeric; 
    DECLARE car_cursor CURSOR FOR SELECT license_plate, last_location FROM 
    vehicle;
    OPEN car_cursor; 
    car_loop: LOOP
        FETCH car_cursor INTO car_temp, loc_temp;
        SET car_lat = (SELECT latitude FROM location WHERE location_id = 
        loc_temp);
        SET car_long = (SELECT longitude FROM location WHERE location_id = 
        loc_temp);
        SET dist = (SELECT ST_Distance_Sphere(
            point(loc_lat, loc_long),
            point(car_lat, car_long)
            ) * .000621371192);
        IF dist < CUR_DIST THEN 
            SET closest_car = car_temp;
            SET CUR_DIST = dist; 
        END IF; 
    END LOOP;
    CLOSE car_cursor; 
    RETURN(closest_car);
END $$ 
DELIMITER ;

现在，这没有返回任何内容，我不太清楚为什么。我对 SQL 还很陌生，所以在此先感谢！

Answer 1

我的建议是首先尝试基于集合的方法，看看它的效果如何。

Some good insights on why found here.

一般策略：对于每个location_id，找到下一个最接近的location_id

一些具体的战术：

• 使用jpgunter'sGETDISTANCE函数（代码如下）
• Click here 在上下文中查看他的代码
• 使用 location 表的自联接计算所有 location_id 值之间的距离
• 确定“太远”的任意距离并排除超出该距离的所有内容
• 这有助于提高性能
• 选择与结果的最小距离

这是自联接到location 表和“太远”标准的启动脚本...

SELECT l1.location_id as l1_location_id
      ,l1.latitude as l1_latitude
      ,l1.longitude as l1_longitude
      ,l2.location_id as l2_location_id
      ,l2.latitude as l2_latitude
      ,l2.longitude as l2_longitude
      ,GETDISTANCE(l1.latitude, l1.longitude, l2.latitude, l2.longitude) as l1_12_distance
  FROM location AS l1
  JOIN location AS l2 ON l1.location_id <> l2.location_id
 WHERE GETDISTANCE(l1.latitude, l1.longitude, l2.latitude, l2.longitude) <= 1000; -- JJAUSSI: arbitrary "too far"

这是jpgunter'sGETDISTANCE 函数...

DELIMITER $$
/*
Takes two latitudes and longitudes in degrees. You could comment out the conversion if you want to pass as radians.
Calculate the distance in miles, change the radius to the earth's radius in km to get km.
*/

DROP FUNCTION IF EXISTS GETDISTANCE$$
CREATE FUNCTION GETDISTANCE 
  (deg_lat1 FLOAT, deg_lng1 FLOAT, deg_lat2 FLOAT, deg_lng2 FLOAT) 
  RETURNS FLOAT 
  DETERMINISTIC 
BEGIN 
  DECLARE distance FLOAT;
  DECLARE delta_lat FLOAT; 
  DECLARE delta_lng FLOAT; 
  DECLARE lat1 FLOAT; 
  DECLARE lat2 FLOAT;
  DECLARE a FLOAT;

  SET distance = 0;

  /*Convert degrees to radians and get the variables I need.*/
  SET delta_lat = radians(deg_lat2 - deg_lat1); 
  SET delta_lng = radians(deg_lng2 - deg_lng1); 
  SET lat1 = radians(deg_lat1); 
  SET lat2 = radians(deg_lat2); 

  /*Formula found here: http://www.movable-type.co.uk/scripts/latlong.html*/
  SET a = sin(delta_lat/2.0) * sin(delta_lat/2.0) + sin(delta_lng/2.0) * sin(delta_lng/2.0) * cos(lat1) * cos(lat2); 
  SET distance = 3956.6 * 2 * atan2(sqrt(a),  sqrt(1-a)); 

  RETURN distance;
END$$
DELIMITER ;

您可能需要考虑修改字段名称year，因为它是reserved word。

如果/当您的数据库增长时，您可能会发现这些名称过于笼统：

• location
• address
• latitude
• longitude

但是，我不知道你的数据库。可能是您的表和字段名称非常适合您的需要。希望这会有所帮助！