Mysql 如何从同一表中的子查询更新列？

出于缓存原因，我需要将Km字段写回位置表，到目前为止，我能够计算Km。到目前为止，我尝试使用联合、子查询等更新的所有内容都会导致SQL错误1093

在MySQL中，如何将Km值更新回表中

这是表模式

CREATE TABLE `locations` (
  `epoch_date` int(10) unsigned DEFAULT NULL,
  `latitude` float NOT NULL,
  `longitude` float NOT NULL,
  `km` float DEFAULT NULL
);

INSERT INTO `locations` (`epoch_date`, `latitude`, `longitude`, `km`) VALUES
(1429913506, -8.7285, 119.401, NULL),
(1429913631, -9.1279, 117.67, NULL),
(1429945707, -8.7063, 119.36, NULL),
(1431929523, -8.5745, 119.707, NULL),
(1431941343, -8.5773, 119.713, NULL),
(1431958383, -8.5881, 119.724, NULL),
(1431969963, -8.589, 119.728, NULL),
(1431998403, -8.5766, 119.724, NULL);

这是生成公里数的select查询

SELECT
    latitude
    , longitude
    , epoch_date
    , @latitude2 :=
    (
        SELECT
            latitude
        FROM locations loc1
        WHERE
            loc1.epoch_date < loc.epoch_date
        ORDER BY epoch_date DESC LIMIT 1 OFFSET 0
    ) as prev_latitude
    , @longitude2 :=
    (
        SELECT
            longitude
        FROM locations loc1
        WHERE
            loc1.epoch_date < loc.epoch_date
        ORDER BY epoch_date DESC LIMIT 1 OFFSET 0
    ) as prev_longitude
    , (ACOS(COS(RADIANS(90-latitude)) *COS(RADIANS(90-@latitude2)) +SIN(RADIANS(90-latitude)) *SIN(RADIANS(90-@latitude2)) *COS(RADIANS(longitude-@longitude2))) *6371) as km
FROM locations loc
ORDER BY epoch_date

---

这里有一个指向SQL FIDLE的链接首先，在select语句中使用变量不能保证有效。MySQL不保证select中表达式的顺序，因此变量可以按任意顺序分配。取决于评估的顺序是危险的

可能通过使用子查询解决该问题后，可以使用子查询更新km列：

update locations l join
       (<your query here>) u
       on l.latitude = u.latitude and l.longitude = u.longitude
    set l.km = u.km;

---

哦，我还看到了其他危险的东西。您正在使用浮点数表示lat和long。您应该真正使用小数，通常情况下，小数10、6就足够了。

您可以设置函数来返回上一个纬度和前一个经度，并在更新查询中使用它

像这样的

DELIMITER $$
CREATE FUNCTION prev_latitude(epoch INT)
  RETURNS FLOAT
  LANGUAGE SQL
BEGIN
  DECLARE lat FLOAT;
  SET lat = null;

  SELECT latitude INTO lat
  FROM locations
  WHERE epoch_date < epoch
  ORDER BY epoch_date DESC LIMIT 1 OFFSET 0;

  RETURN lat;
END;
$$
DELIMITER ;

可能重复的

DELIMITER $$
CREATE FUNCTION prev_longitude(epoch INT)
  RETURNS FLOAT
  LANGUAGE SQL
BEGIN
  DECLARE lon FLOAT;
  SET lon = null;

  SELECT longitude INTO lon
  FROM locations
  WHERE epoch_date < epoch
  ORDER BY epoch_date DESC LIMIT 1 OFFSET 0;

  RETURN lon;
END;
$$
DELIMITER ;

UPDATE locations
SET km = (ACOS(COS(RADIANS(90-latitude)) *COS(RADIANS(90-prev_latitude(epoch_date))) +SIN(RADIANS(90-latitude)) *SIN(RADIANS(90-prev_latitude(epoch_date))) *COS(RADIANS(longitude-prev_longitude(epoch_date)))) *6371);