MySQL 按顺序计算变化

Question

我正在尝试找到一种方法来计算有序列表中值更改的次数。给定一个序列，例如 A, B, A, A, B, C, C，将有 4 个变化，忽略第一个。

我需要的是在 ORDER BY y 之后应用 COUNT(GROUP BY x) 命令之类的东西，但这不是有效的语法。有没有一种简单的方法可以在纯 MySQL 中完成此操作，还是我最好使用 Python 进行迭代？

table 将按日期字段排序。例如：

+------------+-------+
| Date       | Value |
+------------+-------+
| 2015-09-01 | A     |
| 2015-09-02 | B     | (change)
| 2015-09-03 | A     | (change)
| 2015-09-05 | A     |
| 2015-09-06 | B     | (change)
| 2015-09-07 | C     | (change)
| 2015-09-08 | C     |
+------------+-------+

Answer 1

计划

order by Date

calculate lag variable over the ordering

calculate changes ( comparing to lag variable )

take max(changes) and subtract one for ( for the first transition.. )

查询

set @chgs := 0;
set @lag  := null;

select max(chgs) - 1 as num_changes
from
(
  select `Date`, `Value`,
  @chgs := if(@lag = `Value`, @chgs, 
                if(@lag := `Value`, @chgs + 1, @chgs + 1)) as chgs
  from sequence
  order by `Date`
) calc
;

输出

+-------------+
| num_changes |
+-------------+
|           4 |
+-------------+

sqlfiddle

MySQL 按顺序计算变化

MySQL count changes in sequence

mysql

count