Mysql 发现负面变化并据此采取行动

我有一张桌子，我只想看看AB

ID     CODE       COUNT   
102    AB         9
101    AB         8
100    AC         23    //not important!!!!
99     AB         7
98     AB         6
97     AB         5
96     AB         0

然后我想计算代码为“AB”的特定ID之间的差异

所以

我是通过@bonCodigo的查询来实现的

select ID, DIFFERENCE, 
COUNT from (
    SELECT
    t.ID, t.CODE, t.COUNT,
    @PREVCOUNT,
    @PREVCOUNT - t.COUNT DIFFERENCE,
    @PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                           -- must come last!
    FROM
    (SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
    (SELECT @PREVCOUNT := NULL) _uv
    group by t.id, t.code
    )x
where x.difference >= 0
order by ID DESC;

由于我的新输入数据有时会重置计数，因此它从0开始计数，直到任何值

因此，有时我会按以下顺序获取数据：

ID        COUNT
1.        0
2.        1
3.        2
4.        7
5.        4     // which means the counter has reset to 0 and counted up to 4 again.
6.        5

现在我的查询所做的是，它只计算积极的变化，并将其视为差异

那么它的作用是：

Step 1: 1 - 0 = 1
Step 2: 2 - 1 = 1
Step 3: 7 - 2 = 5
Step 4: 4 - 7 = -3 //discarded as this difference is smaller than 0
Step 5: 5 - 4 = 1

如果我把这个加起来，我得到8

而我希望这段代码在出现负差异时从0开始计数

所以我想要的是：

Step 1: 1 - 0 = 1
Step 2: 2 - 1 = 1
Step 3: 7 - 2 = 5
Step 4: 4 - 7 = -3  BUT MAKE IT 4 because the counter started from 0 again.
Step 5: 5 - 4 = 1

所以，如果我把这个加起来，我得到12，一个if语句就可以了

IF((@PREVCOUNT - t.COUNT) < 0, @PREVCOUNT, (@PREVCOUNT - t.COUNT)) DIFFERENCE

这是工作小提琴：

修改后的查询为：

select ID, DIFFERENCE, 
COUNT from (
    SELECT
    t.ID, t.CODE, t.COUNT,
    @PREVCOUNT,
    IF((@PREVCOUNT - t.COUNT) < 0, @PREVCOUNT, (@PREVCOUNT - t.COUNT)) DIFFERENCE,
    @PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                           -- must come last!
    FROM
    (SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
    (SELECT @PREVCOUNT := NULL) _uv
    group by t.id, t.code
    )x
where x.difference >= 0
order by ID DESC;

---

复杂而不清楚。我建议把它分成更简单的部分、部分或多个问题。对不起，下次我会尽量说得更清楚。嗨@Lazykiddy，如果有帮助，请告诉我。谢谢！这就是我需要的。

select ID, DIFFERENCE, 
COUNT from (
    SELECT
    t.ID, t.CODE, t.COUNT,
    @PREVCOUNT,
    IF((@PREVCOUNT - t.COUNT) < 0, @PREVCOUNT, (@PREVCOUNT - t.COUNT)) DIFFERENCE,
    @PREVCOUNT := t.COUNT  -- Updates for the next iteration, so it
                           -- must come last!
    FROM
    (SELECT ID, CODE, COUNT FROM some_table WHERE CODE = 'AB' ORDER BY ID DESC) t,
    (SELECT @PREVCOUNT := NULL) _uv
    group by t.id, t.code
    )x
where x.difference >= 0
order by ID DESC;