如何将表格从行展平到列答案

【问题标题】：How to flatten a table from row to columns如何将表格从行展平到列
【发布时间】：2019-11-15 08:33:10
【问题描述】：

我使用 MariaDB 10.2.21 我没有在其他地方看到过这种确切的情况，因此我请求帮助。

我有一个历史记录表，其中包含对 JIRA 问题中任何字段的每次更改的一条记录：

+----------+---------------+----------+-----------------+---------------------+
| IssueKey | OriginalValue | NewValue |      Field      |     ChangeDate      |
+----------+---------------+----------+-----------------+---------------------+
| HRSK-184 | (NULL)        |        2 | Risk Detection  | 2019-10-24 10:57:27 |
| HRSK-184 | (NULL)        |        2 | Risk Occurrence | 2019-10-24 10:57:27 |
| HRSK-184 | (NULL)        |        2 | Risk Severity   | 2019-10-24 10:57:27 |
| HRSK-184 | 2             |        4 | Risk Detection  | 2019-10-25 11:54:07 |
| HRSK-184 | 2             |        6 | Risk Detection  | 2019-10-25 11:54:07 |
| HRSK-184 | 2             |        3 | Risk Severity   | 2019-10-24 11:54:07 |
| HRSK-184 | 6             |        5 | Risk Detection  | 2019-10-26 09:11:01 |
+----------+---------------+----------+-----------------+---------------------+

每条记录都包含旧值和新值以及已更改的字段类型（“字段”），当然还有该更改的相应时间戳。

我想查询时间点状态，为我提供“风险严重性、风险发生和风险检测”每个字段的最新值组合。

结果应该是这样的：

+----------+----------------+-------------------+------------------+----------------------+
| IssueKey | Risk Severity  |  Risk Occurrence  |  Risk Detection  |  ChangeDate          |
+----------+----------------+-------------------+------------------+----------------------+
| HRSK-184 | 3              |  2                |  5               |  2019-10-26 09:11:01 |
+----------+----------------+-------------------+------------------+----------------------+

有什么想法吗？我被困住了... 提前感谢您的努力！

【问题讨论】：

标签： sql merge pivot mariadb mariadb-10.2

【解决方案1】：

你冷使用了几个内联查询

select 
    IssueKey,
    (
        select t1.NewValue 
        from mytable t1 
        where t1.IssueKey = t.IssueKey and t1.Field = 'Risk Severity'
        order by ChangeDate desc limit 1
    ) `Risk Severity`,
    (
        select t1.NewValue 
        from mytable t1 
        where t1.IssueKey = t.IssueKey and t1.Field = 'Risk Occurrence'
        order by ChangeDate desc limit 1
    ) `Risk Occurrence`,
    (
        select t1.NewValue 
        from mytable t1 
        where t1.IssueKey = t.IssueKey and t1.Field = 'Risk Detection'
        order by ChangeDate desc limit 1
    ) `Risk Severity`,
    max(ChangeDate) ChangeDate
from mytable t
group by IssueKey

使用(IssueKey, Field, ChangeDate, NewValue) 上的索引，这应该是一个有效的选择。

Demo on DB Fiddle：

问题密钥 |风险严重性 |风险发生 |风险严重性 |改变日期 :------- | ------------: | --------------: | ------------: | :----------------- HRSK-184 | 3 | 2 | 5 | 2019-10-26 09:11:01

【讨论】：

这确实是我想要的。我会再调整一下。谢谢@GMB！

【解决方案2】：

MariaDB 10.2 引入了一些 Window Functions 用于分析查询。

其中一个是RANK() OVER (PARTITION BY ...ORDER BY...) 函数。

首先，您可以应用它，然后通过Conditional Aggregation 旋转：

SELECT IssueKey,
       MAX(CASE WHEN Field = 'Risk Severity'   THEN NewValue END ) AS RiskSeverity,
       MAX(CASE WHEN Field = 'Risk Occurrence' THEN NewValue END ) AS RiskOccurrence,
       MAX(CASE WHEN Field = 'Risk Detection'  THEN NewValue END ) AS RiskDetection,
       MAX(ChangeDate) AS ChangeDate
  FROM
  (
   SELECT RANK() OVER (PARTITION BY IssueKey, Field ORDER BY ChangeDate Desc) rnk,
          t.*
     FROM mytable t 
  ) t
  WHERE rnk = 1
  GROUP BY IssueKey;

IssueKey | RiskSeverity  | RiskOccurrence  | RiskDetection  | ChangeDate     
-------- + --------------+-----------------+----------------+--------------------
HRSK-184 |             3 |               2 |              5 | 2019-10-26 09:11:01

Demo

【讨论】：