CODEWITHSUNDEEP
sql
Michael
Michael

Reputation: 42050

How to write an SQL query for this use case?

Suppose I've got a table with three fields: event_name, event_timestamp event_result. Each row is an occurrence of an event event_name occurred at event_timestamp with some event_result. For example:

 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

Now I need a SQL query to select rows with the latest event_timestamp for every event_name within given range of event_timestamp.

How would you write such a query ? I guess it should be standard SQL that works with any reasonable SQL database.

Upvotes: 1

Views: 104

Answers (6)

Yogesh Sharma
Yogesh Sharma

Reputation: 50163

Use subquery :

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

And, in general the most effective method is to use ranking functions (if support)

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;

Upvotes: 2

Hasan Gholamali
Hasan Gholamali

Reputation: 633

this code work for 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

Upvotes: 0

Elroy Jetson
Elroy Jetson

Reputation: 968

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;

Upvotes: 0

Sebastian Siemens
Sebastian Siemens

Reputation: 2421

you can use group by and include the max event_timestamp In MSSQL:

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

Upvotes: 0

HoneyBadger
HoneyBadger

Reputation: 15140

The most portable solution would probably be something like:

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

Upvotes: 1

apomene
apomene

Reputation: 14389

Something like:

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

Upvotes: 0

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