MySQL Group By Consecutive Rows

Question

I have a feed application that I am trying to group results from consecutively. My table looks like this:

    postid | posttype | target | action     |              


        

Answer 1

There is no primary key in your table so for my example I used date. You should create an auto increment value and use that instead of the date in my example.

This is a solution (view on SQL Fiddle):

SELECT 
  postid,
  posttype,
  target,
  action,
  COALESCE((
    SELECT date
    FROM t t2
    WHERE t2.postid = t.postid
    AND t2.posttype = t.posttype
    AND t2.action = t.action
    AND t2.date > t.date
    AND NOT EXISTS (
      SELECT TRUE
      FROM t t3
      WHERE t3.date > t.date
      AND t3.date < t2.date
      AND (t3.postid != t.postid OR t3.posttype != t.posttype OR t3.action != t.action)
    )
  ), t.date) AS group_criterion,
  MAX(title),
  GROUP_CONCAT(content)
FROM t
GROUP BY 1,2,3,4,5
ORDER BY group_criterion

It basically reads:

For each row create a group criterion and in the end group by it.
This criterion is the highest date of the rows following the current one and having the same postid, posttype and action as the current one but there may be not a row of different postid, posttype or action between them.
In other words, the group criterion is the highest occurring date in a group of consecutive entries.

If you use proper indexes it shouldn't be terribly slow but if you have a lot of rows you should think of caching this information.