如何为此用例编写SQL查询？

Question

假设我有一个包含三个字段的表： event_name ， event_timestamp event_result 。 每行都是事件 event_name发生，发生在event_timestamp带有一些event_result 。 例如：

 event_name | event_timestamp  | event_result
 -------------------------------------------
 event_a    | 2018-04-29 00:00 | result_x1
 event_a    | 2018-04-29 00:00 | result_x2
 event_b    | 2018-04-29 00:00 | result_x1
 event_a    | 2018-04-29 00:00 | result_x3
 event_b    | 2018-04-29 00:00 | result_x3

现在，我需要一个SQL查询来为给定event_timestamp范围内的每个 event_name选择具有最新 event_timestamp行。

您将如何编写这样的查询？ 我猜应该是可以与任何合理的SQL数据库一起使用的标准SQL。

Answer 1

使用subquery ：

select * 
from table t
where event_timestamp = (select max(event_timestamp) 
                         from table 
                         where event_name = t.event_name);

而且，通常最有效的方法是使用排名功能（如果支持）

select * from (
      select *,
             row_number() over (partition by event_name order by event_timestamp desc) Seq 
      from table t
      where  event_timestamp between @start and @end
) t
where Seq = 1;

Answer 2

最可移植的解决方案可能是：

SELECT    E.*
FROM      Event E
INNER JOIN (
             SELECT  event_name 
             ,       MAX(event_timestamp) MAX_DT
             FROM    Event 
             WHERE   event_timestamp <= @SomeDate
                  AND event_timestamp > @SomeOtherDate
             GROUP BY event_name 
           ) MAXED
        ON MAXED.event_name = E.event_name 
        AND MAXED.MAX_DT = E.event_timestamp

Answer 3

就像是：

SELECT event_name
    ,MAX(event_timestamp)
FROM myTable
WHERE event_timestamp BETWEEN date1
        AND date2
GROUP BY event_name

Answer 4

您可以在MSSQL中使用group by并包括最大event_timestamp：

select
event_name
, MAX(event_timestamp)
from [table_name]
where event_timestamp between '2018-04-20' and '2018-04-30'
group by event_name

Answer 5

SELECT * FROM `your_table` 
WHERE `event_timestamp` BETWEEN 'your_start_range'AND 'your_end_range'
GROUP BY `event_name`
ORDER BY `event_timestamp` DESC
LIMIT 1;

Answer 6

此代码适用于ms sql

SELECT *
FROM
    (SELECT
        *, ROW_NUMBER() OVER(PARTITION BY event_name ORDER BY event_timestamp DESC) AS row_no
     FROM    Event 
     WHERE
        event_timestamp BETWEEN @BeginDate AND @EndDate) as tbl
WHERE row_no = 1