How do I get distinct rows by a column? - php

Question

I have a huge number of rows that I'd like to get say, last 5 records inserted in that database from 10 different users. If the same user inserted the last 3 rows into database, we must get one row, skip the others two and move to get a row per user, until it count up to 5.
A database like that:
user_id | news_id | title
1 | 1 | foo-1
2 | 2 | foo-2
3 | 3 | foo-3
1 | 4 | baa
4 | 5 | baa0
5 | 6 | baa1
5 | 7 | baa2
6 | 8 | baa3
7 | 9 | baa4
Should return:
user_id | news_id | title
1 | 1 | foo-1
2 | 2 | foo-2
3 | 3 | foo-3
4 | 5 | baa0
5 | 6 | baa1
The current filter was done by PHP, like this:
$used = array();
while ($data = mysql_fetch_array($query)) {
$uid = $data['user_id'];
if(in_array($uid, $used))
continue;
array_push($used, $uid);
// do something with data
}
But I want to refactor it, and do the filter purely by mysql, if possible. I don't know much MySql and that's why I'm having problem to archive this...
Here's what I've tried
select DISTINCT(user_id), news_id, title from XXX
WHERE GROUP BY (news_id) DESC
LIMIT 0,5
How can I do that?

Answer 1

1 way you can do it is to generate a partitioned row number per user and then select 5 records where RowNumber = 1.
SELECT *
FROM
(
SELECT
d.user_id
,d.news_id
,d.title
,(#rn:= if(#uid = user_id, #rn + 1,
if(#uid:=user_id,1,1)
)
) as RowNumber
FROM
Data d
CROSS JOIN (SELECT #uid:=-1, #rn:=0) vars
ORDER BY
user_id
,news_id
) t
WHERE
t.RowNumber = 1
ORDER BY news_id
LIMIT 5;
http://rextester.com/JRIZI7402 - example to show it working
Note you can change the row order by simply changing the ORDER BY statement of the derived table so if you have a column that will signify the latest record e.g. an identity column or a datetime column you can use that, but user_id must be the first criteria to be partitioned correctly.

Answer 2

Do it from your query.
"SELECT * FROM table GROUP BY user_id ORDER BY news_id DESC LIMIT 5"

Answer 3

well, i think this will achieve what you are after.
select user_id, news_id, title from tableName
GROUP BY user_id
ORDER BY news_id DESC
LIMIT 0,5
Hope this helps!