Postgres：简化SQL查询以摆脱子选择

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，创建请求然后完成请求时，我们将有两个事件：

• 请求＃42已创建
• 请求＃42已完成

在上面的示例中，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。

在该视图上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 ，但是没有得到任何好的结果。 到目前为止，我最大的尝试是：

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';

对我来说，这似乎很微不足道，也可能效率不高。

您能提出更好的选择吗？ 我正在使用postgres，很高兴使用特定于postgres的功能。

Answer 1

您正在寻找枢纽查询：

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;

演示

Answer 2

您可以使用窗口功能：

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;

Answer 3

您可以尝试使用大小写和（伪）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