选择值更改 [英] Select on value change

问题描述

在 MySQL 数据库中，我有一个看起来像这样的表:

I have a table that looks like this in a MySQL database:

CREATE TABLE IF NOT EXISTS Example(Batch_Num int, Time DATETIME);
INSERT INTO Example 
VALUES
(1,'2020-12-04 05:06:12'), 
(1,'2020-12-04 05:06:13'), 
(1,'2020-12-04 05:06:14'), 
(2,'2020-12-04 05:06:20'), 
(2,'2020-12-04 05:07:12'), 
(2,'2020-12-04 05:07:20'), 
(1,'2020-12-04 05:07:25'), 
(1,'2020-12-04 05:07:35'), 
(3,'2020-12-04 05:07:35');

我想选择 Batch_Num 与上一个值不同的所有行，包括第一个:

I would like to select all lines where the Batch_Num is different from the previous value including the first one:

+----------+-----------------------+
| BatchNum |      Change_Time      |
+----------+-----------------------+
|        1 | '2020-12-04 05:06:12' |
|        2 | '2020-12-04 05:06:20' |
|        1 | '2020-12-04 05:07:25' |
|        3 | '2020-12-04 05:07:35' |
+----------+-----------------------+

是否有关键字可以访问上一行以将其与当前行进行比较?还是通过其他方式将一条线与其之前的线进行比较?

Is there a keyword maybe to access the previous line to compare the to the current line? Or some other way to compare a line to the line before it?

推荐答案

这是种空缺"问题.孤岛是具有相同 batchnum 的相邻行，您需要每个孤岛的开始.

This is a kind of gaps-and-islands problem. Islands are adjacent rows that have the same batchnum, and you want the start of each island.

在这里，最简单的方法可能是 lag():

Here, the simplest approach is probably lag():

select *
from (
    select e.*,
        lag(batchnum) over(order by time) lag_batchnum
    from example e
) e
where not lag_batchnum <=> batchnum

请注意，这需要MySQL 8.0.在早期版本中，一种替代方法是使用相关子查询:

Note that this requires MySQL 8.0. In earlier versions, one alternative uses a correlated subquery:

select e.*
from example e
where not batchnum <=> (
    select e1.batchnum
    from example e1
    where e1.time < e.time
    order by e1.time desc
    limit 1
)

这是 DB Fiddle上的演示 .

Here is a demo on DB Fiddle.

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