sql - Postgres : Simplify SQL query to get rid of subselects

Question

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:

• 请求 #42 已创建
• 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 View ，以便每个 entity_id 都与创建和完成/失败时间相关联。

对该 View 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 |

我尝试使用 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:

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，而且可能也很低效。

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 特定的功能。

Answer 1

您可以使用窗口函数:

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;