How can I get ordered distinct IDs with pagination? - sql

Question

Let's say I have two tables: Person and Address. Both have a numeric 'id' column, and a person record can have multiple addresses (foreign key 'Address.person_id' which references 'Person.id').
I now want to
search persons with criteria on both the person and it's addresses
sort the result by person/address attributes, and
return the distinct person ids
using pagination (additional restriction on row range, calculated by page number and page size)
Getting the non-distinct person ids is quite simple:
select p.id from person p
left join address a on a.person_id = p.id
where p.firstname is not null
order by a.city, p.lastname, p.firstname
But now I can't just select the distinct(p.id), as I have an order, which cannot be applied unless I select the order criteria as well.
If I wrap the SQL-snippet above with select distinct(id) from (...), I get the distinct ids, but lose the order (ids come in arbitrary order, probably due to hashing)
I came up with a generic but rather impractical solution which works correctly doesn't satisfy me yet (3 outer selects):
select id from (
select id, rownum as r from (
select distinct(ID), min(rownum) from (
select p.id from person p
left join address a on a.person_id = p.id
where p.firstname is not null
order by a.city, p.lastname, p.firstname
)
group by (id)
order by min(rownum)
)
) where r>${firstrow} and r<=${lastrow}
(Placeholders ${firstrow} and ${lastrow} will be replaced by values calculated from page number and page size)
Is there a better way to just get the ordered distinct IDs with
pagination?
I'm implementing these searches using the Hibernate Criteria API, can I somehow realize the outer selects as a Projection in Hibernate, or create my own projection implementation which does this?

Answer 1

you basically want to sort the persons by their min address (not sure this makes any sense to me, but it should only make sense to you). in this case you can try
select person_id
from (
select a.person_id , min(a.city || p.lastname || p.firstname)
from person p left join address a
on (a.person_id = p.id)
where p.firstname is not null
group by a.person_id
order by 2 )
where rownum < x
couple of technical notes -
if every person has an adress lose the left join.
if you'r using group by you dont need to specify distinct.