Skip site navigation (1) Skip section navigation (2)

Re: Joining time fields?

From: "Oliveiros d'Azevedo Cristina" <oliveiros(dot)cristina(at)marktest(dot)pt>
To: "James David Smith" <james(dot)david(dot)smith(at)gmail(dot)com>
Cc: <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Joining time fields?
Date: 2012-07-25 11:08:49
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-novice
Hello again, James,

I got no feedback from you, but I believe this query is flawed.

Also, I believe you need the other fields from your tables and not just the TIMESTAMP fields.

So, considering your tables have fields like table_one.col_on_one and table_two.col_on_two,
I'd try this query

SELECT x."col_on_one", x."date_time",y."col_one_two",y."date_time"
SELECT a."col_on_one",a."date_time", MIN((a."date_time" - b."date_time") * sign(EXTRACT(EPOCH FROM (a."date_time" - b."date_time")))) as dist,
sign(EXTRACT(EPOCH FROM (MIN(a."date_time" - b."date_time")))) as sgn
FROM table_one a,table_two b
GROUP by a."col_on_one",a."date_time"
) x
INNER JOIN t_table_two y
ON y."date_time" = x."date_time" - (sgn*dist )

If it doesn't work, tell me the error it reported and we'll try to fix it


  ----- Original Message ----- 
  From: Oliveiros d'Azevedo Cristina 
  To: James David Smith 
  Cc: pgsql-novice(at)postgresql(dot)org 
  Sent: Tuesday, July 24, 2012 5:20 PM
  Subject: Re: [NOVICE] Joining time fields?

  Hi again,

  This is untested code. Can you see if it works?
  The trouble is that if you have giant tables it will become slow...


  SELECT date_time_in_a, d.date_time as date_time_in_b
  SELECT a.date_time as date_time_in_a, MIN(a.date_time - b.date_time) as dist
  FROM table_one a, table_two b
  GROUP BY a.date_time
  ) c
  table_two d
  ON c.dist - c.date_time_in_a = d.date_time

    ----- Original Message ----- 
    From: James David Smith 
    To: Oliveiros d'Azevedo Cristina 
    Cc: pgsql-novice(at)postgresql(dot)org 
    Sent: Tuesday, July 24, 2012 4:33 PM
    Subject: Re: [NOVICE] Joining time fields?

    Hi Oliveiros,

    Thanks for your time. It's an inner join then hey instead of a left join? Ok, thanks.

    In your example of using one record inn table A, and two records in table B that are exactly the same, I would like the result to be a new table with two records in it. I'm happy for the result of the query to duplicate records from table A.

    Yes, even if the nearest time is 100 years away I would still like the query to get the right result. There is no limit to how far the 'nearest' time is.

    Thank you


    On 24 July 2012 16:25, Oliveiros d'Azevedo Cristina <oliveiros(dot)cristina(at)marktest(dot)pt> wrote:

      Hi, James,

      But that wouldn't be a LEFT JOIN, it will be an INNER JOIN.

      Because you'll always be able to join a date from table a with some date from table b even if it is 100 years away...
      If table a has just one record datetime = 2012-1-1 and table b has two records datetime = 2010-1-1 and datetime = 2011-1-1 then you'd be able to join table a with the second of table b' records.

       You won't be able to join only if table b happens to be empty...ain't I right?

      What do you mean by the closest time? Do you have some threshold ? Are they allowed to be arbitrarily far away one from each other?

        ----- Original Message ----- 
        From: James David Smith 
        To: pgsql-novice(at)postgresql(dot)org 
        Sent: Tuesday, July 24, 2012 3:57 PM
        Subject: [NOVICE] Joining time fields?

        Hi all,

        I wonder if someone could help me out please. I've got two tables, both with a TIMESTAMP field. I'd like to do a left join with them. I'd like to take the date_time from table A, and join it with the nearest date_time from table B. Whether the time from B is before or after the time in A doesn't matter, I just want the closest time. I started with the below query, but it only gets me the column from table B if the time stamp exactly matches which is clearly correct. I'm sure that this should be quite easy but I can't figure it out...!




        FROM table_one a

        LEFT JOIN table_two b ON a.date_time = b.date_time




pgsql-novice by date

Next:From: Gregory KotsaftisDate: 2012-07-25 20:57:56
Subject: Is there an answer to the Ultimate Question for PostgreSQL?
Previous:From: Alessandro GagliardiDate: 2012-07-24 22:49:41
Subject: Re: A different approach: Where to learn?

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group