Count Distinct Date MySQL returning one row - mysql

Question

Ok so be ready I'm working on a weird base :
Every table has 3 column only : varchar('Object'),varchar('Property'),varchar('Value')
Here is a fiddle I've build with examples of my tries
http://sqlfiddle.com/#!9/de22eb/1
I need to extract the last time a server was update. But i'm not interested in the server itself it's more about the date. Once I know that there was an update for a date I'm looking to count every updates on that day.
To do so I'm using 2 tables : the server table
CREATE TABLE IF NOT EXISTS `server` (
`name` varchar(80) NOT NULL,
`field` varchar(80) NOT NULL,
`value` varchar(200) NOT NULL
);
And the event table :
CREATE TABLE IF NOT EXISTS `event` (
`name` varchar(80) NOT NULL,
`field` varchar(80) NOT NULL,
`value` varchar(80) NOT NULL
);
Please go watch the fiddle to have an idea of the content.
I want to have a result like this (based on my example) :
Date Number patched
2017-11-14 2
2017-11-04 1
The problem is that I don't know where I'm wrong on my query (I've separated the step for better understanding inside the fiddle) :
Select date_format(d.val, '%Y-%m-%d') as 'Date', COUNT(distinct
date_format(d.val, '%Y-%m-%d')) as 'Number'
FROM (
Select b.serv,b.val
FROM (
Select serv,val FROM (
Select name as serv, value as val FROM event
where field='server_vers' and
value!='None'
order by serv ASC,
val DESC LIMIT 18446744073709551615) a
group by a.serv) b,
server c
where b.serv = c.name and c.field = 'OS' and c.value = 'Fedora'
) d group by date_format(d.val, '%Y-%m-%d');
It's giving me only one row. Adding group by date_format(d.val, '%Y-%m-%d') at the end makes the Count useless. How can I fix that ?
I want to have for each server for a given OS type the last patch date and then sum the result by date.

Answer 1

Is that what you needed ?
SELECT dates.date, COUNT(dates.date) as patch_count
FROM (
SELECT MAX(date_format(event.value, '%Y-%m-%d')) as date
FROM event
JOIN server ON (event.name = server.name)
WHERE (server.field = 'OS' AND server.value = 'Fedora')
GROUP BY event.name ) as dates
GROUP BY 1
ORDER BY 2 DESC
Here's the fiddle :
http://sqlfiddle.com/#!9/de22eb/37/0
Explanation : We get the last date for every server name. That gives a list of last dates. Then we use this as a table, that we can group on to count each different value.

Answer 2

The datetimes are stored as strings. The first ten characters of that string represent the date. So you get the date with left(value, 10).
You get the last update per server by grouping by server and retrieving max(left(value, 10)), because alphabetic order works on 'yyyy-mm-dd'.
select name, max(left(value, 10))
from event
where field = 'server_vers'
and value <> 'None'
group by name
Build up on this to get the count of updates on those last-update dates:
select left(value, 10), count(*)
from event
where field = 'server_vers'
and left(value, 10) in
(
select max(left(value, 10))
from event
where field = 'server_vers'
and value <> 'None'
group by name
)
group by left(value, 10)
order by left(value, 10);