Postgres：简化 SQL 查询以摆脱子选择答案

【问题标题】：Postgres: Simplify SQL query to get rid of subselectsPostgres：简化 SQL 查询以摆脱子选择
【发布时间】：2019-12-16 03:00:15
【问题描述】：

我有一个events 表，其中包含各种创建、完成和失败事件。每个事件都有一个 ID（表中的主键），还有一个“entity_id”，它将多个事件链接在一起。

例如，当一个请求被创建然后完成时，我们会有两个事件：

已创建请求 #42
请求 #42 已完成

在上面的例子中，42 是请求的 entity_id。

CREATE TABLE IF NOT EXISTS events (
    id SERIAL PRIMARY KEY,
    entity_id INTEGER NOT NULL,
    type VARCHAR(255) NOT NULL,
    occurred_at TIMESTAMP NOT NULL
);

INSERT INTO events (entity_id, type, occurred_at) VALUES
(1, 'created', '2019-08-08 11:20:04.791592+00'),
(1, 'completed', '2019-08-08 11:20:05.791592+00'),
(2, 'created', '2019-08-08 11:20:06.791592+00'),
(2, 'failed', '2019-08-08 11:20:07.791592+00'),
(3, 'created', '2019-08-08 11:20:08.791592+00'),
(3, 'completed', '2019-08-08 11:20:09.791592+00');

我想创建该表的视图，以便每个 entity_id 与创建和完成/失败时间相关联。

对该视图的查询应返回以下结果：

 entity_id |         created_at         |        completed_at        |         failed_at          
-----------+----------------------------+----------------------------+----------------------------
         1 | 2019-08-08 11:20:04.791592 | 2019-08-08 11:20:05.791592 | 
         2 | 2019-08-08 11:20:06.791592 |                            | 2019-08-08 11:20:07.791592
         3 | 2019-08-08 11:20:08.791592 | 2019-08-08 11:20:09.791592 |

我尝试了left join，但没有得到任何好的结果。到目前为止，我最好的尝试是这样的：

SELECT
    e.entity_id,
    e.occurred_at as created_at,
    (SELECT occurred_at FROM events WHERE type = 'completed' AND entity_id = e.entity_id) AS completed_at,
    (SELECT occurred_at FROM events WHERE type = 'failed' AND entity_id = e.entity_id) AS failed_at
FROM events e
WHERE e.type = 'created';

这对我来说似乎很不优雅，而且可能效率也很低。

你能推荐一个更好的选择吗？我正在使用 postgres，并且很高兴使用 postgres 特有的功能。

【问题讨论】：

标签： sql postgresql join subquery

【解决方案1】：

您正在寻找数据透视查询：

SELECT
    entity_id,
    MAX(CASE WHEN type = 'created'   THEN occurred_at END) AS created_at,
    MAX(CASE WHEN type = 'completed' THEN occurred_at END) AS completed_at,
    MAX(CASE WHEN type = 'failed'    THEN occurred_at END) AS failed_at
FROM events
GROUP BY
    entity_id
ORDER BY
    entity_id;

Demo

【讨论】：

看起来很有趣，我从没见过这个。感谢分享 dbfiddle 链接，我不知道它会很有用！

【解决方案2】：

你可以使用窗口函数：

SELECT e.*
FROM (SELECT e.entity_id,
             e.occurred_at as created_at,
             MAX(e.occurred_at) FILTER (WHERE type = 'completed') OVER (PARTITION BY e.entity_id) AS completed_at,
             MAX(e.occurred_at) FILTER (WHERE type = 'failed') OVER (PARTITION BY e.entity_id) AS failed_at
      FROM events e
     ) e
WHERE e.type = 'created';

但是，聚合可能更合适：

SELECT e.entity_id,
       MAX(e.occurred_at) FILTER (WHERE type = 'created') as created_at,
       MAX(e.occurred_at) FILTER (WHERE type = 'completed') AS completed_at,
       MAX(e.occurred_at) FILTER (WHERE type = 'failed') AS failed_at
FROM events e
GROUP BY e.entity_id;

【讨论】：

不错！您的第二个建议与蒂姆的回答之间的主要区别是什么？我尝试EXPLAIN 两个查询，它们产生完全相同的结果，至少在这个小数据集上是这样。这个比另一个有什么优势吗？
@aspyct 。 . . Postgres 支持标准的FILTER 子句，而且速度更快一些。 Tim 的答案是我会在其他数据库中使用的答案。

【解决方案3】：

您可以尝试使用大小写和（假）聚合来减少行数

SELECT
    entity_id,
    max(case when  type = 'created' then occurred_at end ) as created_at,
    max(case when  type = 'completed' then occurred_at end)  as completed_at,
    max(case when  type = 'failed' then occurred_at end ) as failed_at,
FROM events 
group by entity_id

【讨论】：