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Re: Joining time fields?

From: James David Smith <james(dot)david(dot)smith(at)gmail(dot)com>
To: "Oliveiros d'Azevedo Cristina" <oliveiros(dot)cristina(at)marktest(dot)pt>
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: Joining time fields?
Date: 2012-07-26 14:22:51
Message-ID: CAMu32ADqrcgSwqnh7fAFq1yRuOoiDXL=o1x4y5Ue9O=-DOhDow@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-novice
Afternoon,

Hmm. If I use INNER JOIN then the rows that have data from table A but not
from table B are not present any more. This isn't correct. I want the rows
from table A to be there. They want joining with the nearest (in time) data
from table B.

To clarify, every row from table A should have a join with table B. The
query should take each row of table A, and then find the nearest time in
table B. If the query finds two records in table B that are both exactly
the same time away from the record in table A (this won't happen very
often), then the record in table A should be duplicated and both records
from table B joined to each of them respectively.

Does that make sense!?

James




On 26 July 2012 15:14, Oliveiros d'Azevedo Cristina <
oliveiros(dot)cristina(at)marktest(dot)pt> wrote:

> **
> Howdy, James,
>
> The line takes into account negative differences, the sgn is for that.
>
> On my test case I actually din't have the situation you described.
>
> But I notice that you are using LEFT JOIN instead of INNER JOIN. Can that
> be the cause for the empty fields?
>
>
>
> Best,
> Oliveiros
>
>
>
> ----- Original Message -----
> *From:* James David Smith <james(dot)david(dot)smith(at)gmail(dot)com>
> *To:* Oliveiros d'Azevedo Cristina <oliveiros(dot)cristina(at)marktest(dot)pt>
> *Cc:* pgsql-novice(at)postgresql(dot)org
> *Sent:* Thursday, July 26, 2012 2:40 PM
> *Subject:* Re: [NOVICE] Joining time fields?
>
> Dear Oliveiros,
>
> Thanks for your reply. I've been working on this today, but don't think
> that the query is quite right. On this line of the query:
>
> //   ON y."date_time" = x."date_time" - (sgn*dist )
>
> I think that we also need to take into account a positive difference as
> well as a negative? I've had a play around with your query and put all my
> proper field names in. I think it works. it returns the number of rows that
> I would expect. The only odd thing is that the first 20-30 rows of data,
> which represent the earliest data_time in table A, do not have any data
> linked to them in table B. Just empty cells. Your thoughts are appreciated!
>
> SELECT
> x."id" as id,
> x."person" as person,
> x."the_geom_osgb36" as location,
> x."date_time" as gps_time,
> y."date_time" as microaeth_time,
> y."bc" as black_carbon
>
> FROM(
>  SELECT
>  a."id",
>  a."date_time",
>  a.person,
>  a.the_geom_osgb36,
>  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 gps_12_07_2012 a, microaeth_12_07_2012 b
>  WHERE a.person = 'Ben Barratt'
>
>  GROUP by
>  a."id",
>  a."date_time",
>  a.person,
>  a.the_geom_osgb36
>
>  ) x
>
> LEFT JOIN (SELECT * FROM microaeth_12_07_2012 WHERE person = 'Ben
> Barratt') y
> ON y."date_time" = x."date_time" - (sgn*dist)
> OR
> y."date_time" = x."date_time" + (sgn*dist)
>
> ORDER BY x.id, x.date_time
>
>
>
>
>
>
>
> On 25 July 2012 12:08, Oliveiros d'Azevedo Cristina <
> oliveiros(dot)cristina(at)marktest(dot)pt> wrote:
>
>> **
>>  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"
>> FROM(
>> 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
>>
>> Best,
>> Oliver
>>
>>
>>
>>  ----- Original Message -----
>> *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
>>  *Sent:* Tuesday, July 24, 2012 5:20 PM
>> *Subject:* Re: [NOVICE] Joining time fields?
>>
>> Hi again,
>> James,
>>
>> This is untested code. Can you see if it works?
>> The trouble is that if you have giant tables it will become slow...
>>
>> Best,
>> Oliveiros
>>
>> SELECT date_time_in_a, d.date_time as date_time_in_b
>> FROM
>> (
>> 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
>> JOIN
>> table_two d
>> ON c.dist - c.date_time_in_a = d.date_time
>>
>>
>> ----- Original Message -----
>> *From:* James David Smith <james(dot)david(dot)smith(at)gmail(dot)com>
>> *To:* Oliveiros d'Azevedo Cristina <oliveiros(dot)cristina(at)marktest(dot)pt>
>> *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
>>
>> James
>>
>>
>>
>>
>> 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?
>>>
>>> Best,
>>> Oliveiros
>>>
>>> ----- Original Message -----
>>> *From:* James David Smith <james(dot)david(dot)smith(at)gmail(dot)com>
>>> *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...!
>>>
>>>
>>> Select
>>>
>>> a.date_time
>>>
>>> b.date_time
>>>
>>> FROM table_one a
>>>
>>> LEFT JOIN table_two b ON a.date_time = b.date_time
>>>
>>>
>>>
>>> Thanks
>>>
>>>
>>>
>>> James
>>>
>>>
>>
>

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