[英]How to update a column from a subquery in the same table?
由于缓存的原因,我需要将Km(公里)字段写回到位置表,到目前为止,我已经能够计算Km。 到目前为止,我尝试的所有操作(使用联合,子查询等更新)都给我SQL错误1093。
在MySQL中,如何将Km值更新回表?
这是表架构
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KM(公里)的选择查询
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
这是SQL Fiddle的链接http://sqlfiddle.com/#!9/7f95de/2/0
首先,不能保证您在select
语句中使用变量。 MySQL不保证select
中表达式的顺序,因此可以按任何顺序分配变量。 根据评估顺序是危险的。
解决该问题后(可能通过使用子查询),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,我还看到其他危险。 您正在使用浮点数表示lat和long。 实际上,您应该使用小数,通常decimal(10, 6)
10,6)这样的decimal(10, 6)
就足够了。
您可以设置函数以返回prev_latitude和pre_longitude并将其用于UPDATE
查询中。
像这样
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 ;
和
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 ;
现在,您的更新查询可以写为
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);
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