SELECT语句的两行之间的MySQL差异 [英] MySQL difference between two rows of a SELECT Statement
问题描述
我正在尝试使mysql数据库中的两行有所不同.
我的这张表包含ID,公里,日期,car_id,car_driver等...
由于我并非总是以正确的顺序在表中输入信息,因此我可能会得到如下信息:
I am trying to make the difference of two rows in an mysql database.
I have this table containing ID, kilometers, date, car_id, car_driver etc...
Since I don't always enter the information in the table in the correct order, I may end up with information like this:
ID | Kilometers | date | car_id | car_driver | ...
1 | 100 | 2012-05-04 | 1 | 1
2 | 200 | 2012-05-08 | 1 | 1
3 | 1000 | 2012-05-25 | 1 | 1
4 | 600 | 2012-05-16 | 1 | 1
使用select语句,我可以对表格进行正确排序:
With a select statement I am able to sort my table correctly:
SELECT * FROM mytable ORDER BY car_driver ASC, car_id ASC, date ASC
我会得到这个:
ID | Kilometers | date | car_id | car_driver | ...
1 | 100 | 2012-05-04 | 1 | 1
2 | 200 | 2012-05-08 | 1 | 1
4 | 600 | 2012-05-16 | 1 | 1
3 | 1000 | 2012-05-25 | 1 | 1
现在,我想查看一下我基本上具有以下额外信息的地方:自上次约会以来的公里数,我想获得以下信息:
Now I would like to make a view where basically I have this extra information: Number of kilometers since last date and I would like to obtain something like this:
ID | Kilometers | date | car_id | car_driver | number_km_since_last_date
1 | 100 | 2012-05-04 | 1 | 1 | 0
2 | 200 | 2012-05-08 | 1 | 1 | 100
4 | 600 | 2012-05-16 | 1 | 1 | 400
3 | 1000 | 2012-05-25 | 1 | 1 | 400
我曾考虑过进行INNER JOIN来执行所需的操作,但由于无法正确排序,我感到无法进行ID上的联接.
有没有办法实现我想要的?
I thought of doing an INNER JOIN to perform what I wanted, but I have the feeling I can't do the join on my ID since they are not sorted correctly.
Is there a way to achieve what I want?
我是否可以创建带有某种row_number的视图,然后可以在我的INNER JOIN中使用它?
Shall I create a view with a sort of row_number that I can then used in my INNER JOIN?
推荐答案
SELECT
mt1.ID,
mt1.Kilometers,
mt1.date,
mt1.Kilometers - IFNULL(mt2.Kilometers, 0) AS number_km_since_last_date
FROM
myTable mt1
LEFT JOIN myTable mt2
ON mt2.Date = (
SELECT MAX(Date)
FROM myTable mt3
WHERE mt3.Date < mt1.Date
)
ORDER BY mt1.date
Sql小提琴
或者,通过MySql hackiness模拟lag()
函数...
Sql Fiddle
Or, by emulating a lag()
function through MySql hackiness...
SET @kilo=0;
SELECT
mt1.ID,
mt1.Kilometers - @kilo AS number_km_since_last_date,
@kilo := mt1.Kilometers Kilometers,
mt1.date
FROM myTable mt1
ORDER BY mt1.date
这篇关于SELECT语句的两行之间的MySQL差异的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!