MySQL query GROUP BY two columns - mysql

Question

I need help with MySQL query
I have this cenary:
table A
- page_id
- rev_id
- date
one page_id can have multiples rev_id
table B
- rev_id
- words
I have what words have in each revision
I need return for each date the quantity of words that I have in the
last rev_id in each page_id
Example:
table A
page_id | rev_id | date
---------------------------------
1 | 231 | 2002-01-01
2 | 345 | 2002-10-12
1 | 324 | 2002-10-13
3 | 348 | 2003-01-01
--
table B
rev_id | words
---------------
231 | 'ask'
231 | 'the'
231 | 'if'
345 | 'ask'
324 | 'ask'
324 | 'if'
348 | 'who'
magical sql here edited to show how its calculated {page_id : [words]}
date | count(words)
--------------------------
2002-01-01 | 3 { 1:[ask, the, if] }
2002-10-12 | 4 { 1:[ask, the, if], 2:[ask] }
2002-10-13 | 3 { 1:[ask, if], 2:[ask] }
2003-01-01 | 4 { 1:[ask, if], 2:[ask], 3:[who] }
I did this query, but my date are fixed and I need for all dates contained in table revision:
SELECT SUM(q)
FROM (
SELECT COUNT(equation) q
FROM revision r, equation e
WHERE r.rev_id in (
SELECT max(rev_id)
FROM revision
WHERE date < '2006-01-01'
GROUP BY page_id
)
AND r.rev_id = e.rev_id
GROUP BY date
) q;
Solved
My friend help-me to create query to solve my problem!
select s.date, count(words) from
(select d.date, r.page_id, max(r.rev_id) as rev_id
from revision r, (select distinct(date) from revision) d
where d.date >= r.date group by d.date, r.page_id) s
join words e on e.rev_id = s.rev_id
group by s.date;

Answer 1

I think this is a basic join and group by:
select a.date, count(*)
from a join
b
on a.rev_id = b.rev_id
group by a.date;
EDIT:
Oh, I think I get it. This is a cumulative thing. That makes it more complicated.
select d.date,
(select count(*)
from a join
b
on a.rev_id = b.rev_id
where a.date <= d.date and
a.rev_id = (select max(a2.rev_id) from a a2 where a2.date = a.date and a2.date <= d.date)
) as cnt
from (select date from a) d;
But that won't work in MySQL because of the nesting of the correlation clause. So, we can restructure the logic as:
select a.date, count(*)
from (select a.*,
(select max(a2.rev_id)
from a a2
where a2.date <= a.date and a2.page_id = a.page_id
) as last_rev_id
from a
) a join
b
on a.last_rev_id = b.rev_id
group by a.date;