How to bypass a reference to an outer table in subquery? - mysql

Question

I've been dealing with these two tables:
Document
id company_id etc
=======================
1 2 x
2 2 x
Version
id document_id version date_created date_issued date_accepted
==========================================================================
1 1 1 2013-04-29 2013-04-30 NULL
2 2 1 2013-05-01 NULL NULL
3 1 2 2013-05-01 2013-05-01 2013-05-03
There's a page where I want to list all documents with their attributes.
And I would like to add a single have status from each document.
The status can be derived from the most present date that corresponding Versions have.
It is possible that an older version is being accepted.
The query result I am looking for is like this:
id company_id etc status
==================================
1 2 x accepted
2 2 x created
I started out by making a query which combines all dates and add a status next to it.
It works as expected and when I add the document_id things look alright.
SELECT `status`
FROM (
SELECT max(date_created) as `date`,'created' as `status` FROM version WHERE document_id = 1
UNION
SELECT max(date_issued),'issued' FROM version WHERE document_id = 1
UNION
SELECT max(date_accepted),'accepted' FROM version WHERE document_id = 1
ORDER BY date DESC
LIMIT 1
) as maxi
When I try to incorporate this query as a subquery, I can't make it work.
SELECT *, (
SELECT `status` FROM (
SELECT max(date_created) as `date`,'created' as `status`FROM version WHERE document_id = document.id
UNION
SELECT max(date_issued),'issued' FROM version WHERE document_id = document.id
UNION
SELECT max(date_accepted),'accepted' FROM version WHERE document_id = document.id
ORDER BY date DESC
LIMIT 1
) as maxi
) as `status`
FROM `document`
This will get me the error Unknown column 'document.id' in 'where clause'. So I've read around at SO and figured it simply can't reach the value offer.id since it's a subquery in a subquery. So I tried to take another approach and get all the statuses at once, to avoid the WHERE statement, and JOIN them. I ended up with the next query.
SELECT MAX(`date`),`status`, document_id
FROM (
SELECT datetime_created as `date`, 'created' as `status`,document_id FROM `version`
UNION
SELECT datetime_issued, 'issued',document_id FROM `version`
UNION
SELECT datetime_accepted, 'accepted',document_id FROM `version`
) as dates
GROUP BY offer_id
No error this time but I realized that the status couldn't be the correct one since it got lost during the GROUP BY. I've tried combinations of the two but both flaws keep hindering me. Could any one suggest how to do this in a single query without changing my database? (I know that saving the dates in a separate table would simply things)

Answer 1

I have not tested this, but you can do it like this (you might need to tweak the details)
It is basically looking at it from a completely different angle.
select
d.*,
(CASE GREATEST(ifnull(v.date_created, 0), ifnull(v.date_issued,0), ifnull(v.date_accepted,0) )
WHEN null THEN 'unknown'
WHEN v.date_accepted THEN 'accepted'
WHEN v.date_issued THEN 'issued'
WHEN v.date_created THEN 'created'
END) as status
from document d
left join version v on
v.document_id = d.document_id and
not exists (select 1 from (select * from version) x where x.document_id = v.document_id and x.id <> v.id and x.version > v.version)

Answer 2

Can you normalise your table designs to move the status / dates onto a different table from the Versions?
If no possibly something like this:-
SELECT Document.id, Document.company_id, Document.etc, CASE WHEN Sub1.status = 3 THEN 'accepted' WHEN Sub1.status = 2 THEN 'issued' WHEN Sub1.status = 1 THEN 'created' ELSE NULL END AS status
FROM Document
INNER JOIN (
SELECT document_id, MAX(CASE WHEN date_accepted IS NOT NULL THEN 3 WHEN date_issued IS NOT NULL THEN 2 WHEN date_created IS NOT NULL THEN 1 ELSE NULL END) AS status
FROM Version
GROUP BY document_id
) Sub1
ON Document.id = Sub1.document_id
The subselect gets the highest status for any document from the version table. Each possible versions highest status is returned as a number, and by grouping that on the document id it will get the highest status of any version. This is joined back against the Document table and the number for the version number converted into the text description.

Answer 3

select Doc.document_id,Doc.company_id,Doc.etc,f.status
from Document Doc
inner join
(select Ver.document_id,
case when Ver.date_accepted is not null then 'Accepted'
when Ver.date_issued is not null then 'Issued'
when Ver.date_created is not null then 'Created'
end as status
from version Ver
inner join (
select document_id,MAX(version) VersionId
from version
group by document_id
)t on t.document_id=Ver.document_id
where t.VersionId=Ver.version
)f on Doc.document_id=f.document_id
SQL Fiddle