MySQL - How to set variable value inside select statement?

Question

I want to get the value of the previous row, current row, and add them. If there's no previous row then I'll just set previous to 0. Here's what I tried.

SET @previous = CASE WHEN (SELECT MAX(value) FROM table WHERE dateTime < '2018-07-01') IS NULL 
THEN 0 
ELSE (SELECT MIN(value) FROM table WHERE dateTime BETWEEN '2018-07-01' AND '2018-07-30') 
END;

SELECT @previous AS Previous, 
@previous:=MAX(value) AS Current, 
(@previous + MAX(value)) AS Joined
FROM table 
WHERE dateTime BETWEEN '2018-07-01' AND '2018-07-30'
GROUP BY dateTime;

Here's the table content:

dateTime   | value 
2018-07-01 | 1
2018-07-01 | 1.2
2018-07-02 | 1.7
2018-07-03 | 2.1
2018-07-05 | 2.6
2018-07-05 | 3
2018-07-06 | 3.5

Unfortunately setting @previous:=MAX(value) does not change @previous value for the next row, if it's 0 in initialization, it will remain 0. Sample Result:

dateTime   | Previous | Current | Joined
2018-07-01 | 0        | 1.2     | 1.2
2018-07-02 | 0        | 1.7     | 1.7
2018-07-03 | 0        | 2.1     | 2.1
2018-07-05 | 0        | 4       | 4
2018-07-06 | 0        | 4       | 4

Expected result

dateTime   | Previous | Current | Joined
2018-07-01 | 0        | 1.2     | 1.2
2018-07-02 | 1.2      | 1.7     | 2.9
2018-07-03 | 1.7      | 2.1     | 3.8
2018-07-05 | 2.1      | 3       | 5.1
2018-07-06 | 3        | 3.5     | 6.5

Hope you can help me!

Answer 1

解决方案是先在子查询中运行GROUP BY语句，然后才运行变量计算：

SELECT dateTime, @previous AS Previous, @previous + MaxVal AS Joined, @previous := MaxVal AS Current FROM ( SELECT `dateTime`, MAX(`value`) AS MaxVal FROM `table` WHERE `dateTime` BETWEEN '2018-07-01' AND '2018-07-30' GROUP BY `dateTime` ) grp ORDER BY `dateTime`