Table joining query with dates

Question

Hello,
I have two tables, one containing a list of pupils, their class start date and their class end dates. The second table contains the address (postcode) history of the pupil. I need to bring these two tables together into one query to show all the classes from the 'pupils' table and add in the most recent postcode of the address prior to the start date of the clasS from the 'addresses' table.  I.e. to show where the pupil lived when they started the class.
Could anyone show me how to do this please? I've provided a 'desired' table to show what I need the results to look like.
I hope this is clear, please ask if it isn't - I'm sure this should be simple! But I can't get my head around it. I'm using Oracle 9i and SQL Developer.
Thanks :)
select pupils.* from
(select 'A227' as pupil_id, to_date('21/04/2010','dd/mm/rrrr') class_start, to_date('21/09/2010','dd/mm/rrrr') class_end from dual union all
select 'A227' as pupil_id, to_date('08/08/2011','dd/mm/rrrr'), to_date('26/10/2011','dd/mm/rrrr') from dual ) pupils
select addresses.* from
(select 'A227' as pupil_id, to_date('20/04/2010','dd/mm/rrrr') address_start, null address_end, 'TW1 XVT' as postcode from dual union all
select 'A227' as pupil_id, to_date('03/08/2011','dd/mm/rrrr'), to_date('31/10/2011','dd/mm/rrrr'), 'TW3 ZE3' as postcode from dual union all
select 'A227' as pupil_id, to_date('01/11/2011','dd/mm/rrrr'), null, 'n23 4ty' as postcode from dual) addresses
select desired.* from
(select 'A227' as pupil_id, to_date('21/04/2010','dd/mm/rrrr') class_start, to_date('21/09/2010','dd/mm/rrrr') class_end, to_date('20/04/2011','dd/mm/rrrr') as address_start, 'TW1 XVT' as postcode from dual union all
select 'A227' as pupil_id, to_date('08/08/2011','dd/mm/rrrr') class_start, to_date('26/10/2011','dd/mm/rrrr') class_end, to_date('03/08/2011','dd/mm/rrrr') as  address_start, 'TW3 ZE3' as postcode from dual) desired
 

Answer 1

Hi,
That's an example of a Top-N Query , and here's one way to do it:
WITH     got_r_num     AS
     SELECT     p.*
     ,     a.address_start
     ,     a.postcode
     ,     ROW_NUMBER () OVER ( PARTITION BY  p.pupil_id
                               ,                    p.class_start
                               ORDER BY          a.address_start     DESC
                       )         AS r_num
     FROM     pupils        p
     JOIN     addresses  a  ON   p.pupil_id        = a.pupil_id
                     AND  p.class_start  >= a.address_start
SELECT     pupil_id, class_start, class_end, address_start, postcode
FROM     got_r_num
WHERE     r_num     = 1
;As you can see, the sib-query pairs each pupil and class with all the earlier addresses, and then numbers the addresses such that the latest address_start gets the lowest r_num (that is, 1, since ROW_NUMBER always starts with 1). The main query displays only the most recent address (the one with r_num = 1) for each pupil and class.
Does addresses.address_end plays any role in this problem? For example, would it matter if the most recent address for a given pupil and class had expired before the class started? That is, if we change the middle of address to:
select  'A227'          as pupil_id
,      to_date ('03/08/2011', 'dd/mm/rrrr')
,      to_date ('07/08/2011', 'dd/mm/rrrr')     -- Not 31/10/2011
,      'TW3 ZE3'      as postcode
from      dual would it still count as the last address before the class that started on 08/08/2011?
Edited by: Frank Kulash on Nov 1, 2011 11:36 AM
Added question about address_end