如何仅选择最近的时间戳？

Question

如果我在多个表上执行内连接，我如何确保结果集只包含最近的时间戳。例如

SELECT
  e.customer_id AS customer_id,
  e.event_id AS event_id,
  #MOST RECENT TIMESTAMP from car.updated_on, motorcycle.updated_on or walkc.updated_on
FROM
  event_table AS e
  INNER JOIN car AS c ON e.customer_id = c.customer_id
  INNER JOIN motorcycle AS m ON e.customer_id = m.customer_id
  INNER JOIN walk AS w ON e.customer_id = w.customer_id
WHERE
  e.event_id = c.event_id
  AND e.event_id = m.event_id
  AND e.event_id = w.event_id

我有一个记录所有发生的事件的表，我只想提取最近的时间戳，而不管所有三个事件（汽车、摩托车或步行）中的所有客户的事件类型。

样本数据：

事件

customer_id	event_id
1	100
2	101
3	102
4	103
5	104
6	105
7	106
8	107
9	108
10	109

汽车

customer_id	event_id	car_id	updated_on
1	100	1	2021-07-23 10:09:05
2	101	1	2021-07-23 10:09:05
3	102	1	2021-07-23 10:09:05
4	103	1	2021-07-23 10:09:05
5	104	1	2021-07-23 10:09:05
6	105	1	2021-07-23 10:09:05
7	106	1	2021-07-23 10:09:05
8	107	1	2021-07-23 10:09:05
9	108	1	2021-07-23 10:09:05
10	109	1	2021-07-23 10:09:05

摩托车

customer_id	event_id	motorcycle_id	updated_on
1	100	1	2021-07-23 10:09:00
2	101	1	2021-07-23 10:09:00
3	102	1	2021-07-23 10:09:00
4	103	1	2021-07-23 10:09:00
5	104	1	2021-07-23 10:09:10
6	105	1	2021-07-23 10:09:10
7	106	1	2021-07-23 10:09:00
8	107	1	2021-07-23 10:09:00

走路

customer_id	event_id	walk_id	updated_on
1	100	1	2021-07-23 10:09:00
2	101	1	2021-07-23 10:09:00
3	102	1	2021-07-23 10:09:00
4	103	1	2021-07-23 10:09:00
5	104	1	2021-07-23 10:09:00
6	105	1	2021-07-23 10:09:00
7	106	1	2021-07-23 10:09:00
8	107	1	2021-07-23 10:09:15
9	108	1	2021-07-23 10:09:15

期望的结果：

customer_id	event_id	updated_on	comment
1	100	2021-07-23 10:09:05	TS from car
2	101	2021-07-23 10:09:05	TS from car
3	102	2021-07-23 10:09:05	TS from car
4	103	2021-07-23 10:09:05	TS from car
5	104	2021-07-23 10:09:10	TS from motorcycle
6	105	2021-07-23 10:09:10	TS from motorcycle
7	106	2021-07-23 10:09:15	TS from walk
8	107	2021-07-23 10:09:15	TS from walk

我不需要最终结果集中的comment，我添加它只是为了解释。实际上，我不在乎事件是什么。我只关心四个表中的INNER JOIN；所以最多应该只有8条记录，我只想要最新（最高）的时间戳值。 customer_id 和 event_id 需要匹配所有 INNER JOINS。

例如：customer_id = 1 和 event_id = 100；这存在于所有 4 个表中。它具有updated_on 的三个值（分别来自：汽车、摩托车和步行）。我想要MAX(2021-07-23 10:09:05, 2021-07-23 10:09:00, 2021-07-23 10:09:00)； MAX(car.updated_on, motorcyle.updated_on, walk.updated_on).

任何帮助将不胜感激，谢谢。

编辑：我在两个查询中得到了想要的结果。希望优化为单个查询。

1. 仅获取三个表之间的UNIQUE 记录并将它们存储在另一个名为event_joined 的位置。此表每次都会被完全覆盖，而不仅仅是附加到。

SELECT
  e.customer_id AS customer_id,
  e.event_id AS event_id,
FROM
  event_table AS e
  INNER JOIN car AS c ON e.customer_id = c.customer_id
  INNER JOIN motorcycle AS m ON e.customer_id = m.customer_id
  INNER JOIN walk AS w ON e.customer_id = w.customer_id
WHERE
  e.event_id = c.event_id
  AND e.event_id = m.event_id
  AND e.event_id = w.event_id

1. 在执行UNION 之前，我们知道所有三个表将具有相同的行数，因为我们之前仅将它们加入以匹配记录。现在我们只需 GROUP BY 并获取 MAX（最近的）时间戳。

SELECT event_temp.customer_id, event_temp.event_id, MAX(event_temp.updated_on) AS updated_on
FROM (
  SELECT customer_id, event_id, updated_on FROM car AS c INNER JOIN event_joined AS ej ON e.customer_id = c.customer_id AND e.event_id = c.event_id
  UNION ALL
  SELECT customer_id, event_id, updated_on FROM motorcycle AS m INNER JOIN event_joined AS ej ON e.customer_id = c.customer_id AND e.event_id = c.event_id
  UNION ALL 
  SELECT customer_id, event_id, updated_on FROM walk AS w INNER JOIN event_joined AS ej ON e.customer_id = c.customer_id AND e.event_id = c.event_id
) AS event_temp
GROUP BY event_temp.customer_id, event_temp.event_id;

有没有办法将其优化为单个查询？谢谢。

Answer 1

您可以使用 CROSS APPLY 进行简单查询，如下所示：

SELECT
  e.customer_id AS customer_id,
  e.event_id AS event_id,
  max(t.updated_On)
FROM
  event_table AS e
  INNER JOIN car AS c ON e.customer_id = c.customer_id and e.event_id = c.event_id
  INNER JOIN motorcycle AS m ON e.customer_id = m.customer_id and e.event_id = m.event_id
  INNER JOIN walk AS w ON e.customer_id = w.customer_id and e.event_id = w.event_id
  CROSS APPLY (values (c.updated_On),(m.updated_On),(w.updated_On)) as t(updated_On)
GROUP BY e.customer_id,
  e.event_id

样本数据和工作解决方案

declare @event table(cust_id int, event_id int)
declare @car table(cust_id int, event_id int, updated_on datetime)
declare @walk table(cust_id int, event_id int, updated_on datetime)

insert into @event values (1, 100)
insert into @car values (1,100, '2020-01-01')
insert into @walk values(1,100, '2020-02-01')

SELECT
  e.cust_id AS customer_id,
  e.event_id AS event_id,
  max(t.updatedON)  as recent_timestamp
FROM
  @event AS e
  INNER JOIN @car AS c ON e.cust_id = c.cust_id and e.event_id = c.event_id
  INNER JOIN @walk AS w ON e.cust_id = w.cust_id and e.event_id = w.event_id
  CROSS APPLY (values(c.updated_On),(w.updated_on)) as t(updatedOn)
  group by e.cust_id, e.event_id

customer_id	event_id	recent_timestamp
1	100	2020-02-01 00:00:00.000