MySQL: Get most recent record satisfying certain conditions - mysql

Question

I was inspired by this post. But what I'm going to solve is more complex.
In the table below we have three columns, id,rating,created, call it test_table,
+----+--------+----------------------+
| id | rating | created |
+----+--------+----------------------+
| 1 | NULL | 2011-12-14 09:25:21 |
| 1 | 2 | 2011-12-14 09:26:21 |
| 1 | 1 | 2011-12-14 09:27:21 |
| 2 | NULL | 2011-12-14 09:25:21 |
| 2 | 2 | 2011-12-14 09:26:21 |
| 2 | 3 | 2011-12-14 09:27:21 |
| 2 | NULL | 2011-12-14 09:28:21 |
| 3 | NULL | 2011-12-14 09:25:21 |
| 3 | NULL | 2011-12-14 09:26:21 |
| 3 | NULL | 2011-12-14 09:27:21 |
| 3 | NULL | 2011-12-14 09:28:21 |
+----+--------+----------------------+
I want to write a query which selects the most recent rating but not null for every id. If all of the ratings are null for a specific id, we select the most recent rating. The desired result is as follows:
+----+--------+----------------------+
| id | rating | created |
+----+--------+----------------------+
| 1 | 1 | 2011-12-14 09:27:21 |
| 2 | 3 | 2011-12-14 09:27:21 |
| 3 | NULL | 2011-12-14 09:28:21 |
+----+--------+----------------------+

Answer 1

The following gets the creation date:
select t.id,
coalesce(max(case when rating is not null then creation_date end),
creation_date
) as creation_date
from t
group by t.id;
You can then do this as:
select t.*
from t
where (id, creation_date) in (select t.id,
coalesce(max(case when rating is not null then creation_date end),
creation_date
) as creation_date
from t
group by t.id
);

Answer 2

One possible answer is this. Create a list of max(create) date per id and id having all NULL rating.
select t1.*
from myTable t1
join (
select id, max(created) as created
from myTable
where rating is not NULL
group by id
UNION ALL
select id, max(created) as created
from myTable t3
where rating is NULL
group by id
having count(*) = (select count(*) from myTable t4 where t4.id=t3.id)
) t2
where t1.id=t2.id
and t1.created=t2.created
order by t1.id;

Answer 3

select a.* from #test a join (select id, max(created) created
from #test
where rating is not null
group by id )b on a.id=b.id and a.created=b.created
union
select a.* from #test a join
(select id, max(created) created
from #test
where rating is null
and id not in
(select id from (select id, max(created) created
from #test
where rating is not null
group by id )d
group by id)
group by id )b on a.id=b.id and a.created=b.created

Answer 4

This query should work:
select a.id, a.rating, b.m from test_table a
join (
select id, max(created) as m from test_table
where rating is not null
group by id
) b on b.id = a.id and b.m = a.created
union
select a.id, a.rating, b.m from test_table a
join(
select id, max(created) as m from test_table a
where not exists
(select 1 from test_table b where a.id = b.id and b.rating is not null)
group by id
)b on b.id = a.id and b.m = a.created

Answer 5

You can get the created value in a correlated LIMIT 1 subquery:
select t.id, (
select created
from mytable t1
where t1.id = t.id
order by rating is null asc, created desc
limit 1
) as created
from (select distinct id from mytable) t
If you also need the rating column, you will need to join the result with the table again:
select t.*
from (
select t.id, (
select created
from mytable t1
where t1.id = t.id
order by rating is null asc, created desc
limit 1
) as created
from (select distinct id from mytable) t
) x
natural join mytable t
Demo: http://sqlfiddle.com/#!9/49e68c/8