How to nest two Queries with Eloquent Laravel - mysql

Question

I do have a table that has multiples visitors in it. In my result list only the last visit of each visitor should be shown.
the table looks like:
id | created_at | descr | website | source
------------------------------------------
2 | 2017_12_22 | john | john1.com | a
3 | 2017_12_23 | marc | ssdff.com | b
4 | 2017_12_24 | john | john1.com | c
5 | 2017_12_24 | tina | def.com | b
6 | 2017_12_25 | stef | abc.com | a
7 | 2017_12_26 | john | john2.com | c
If I do an
->orderBy('visitors.created_at', 'desc')
->groupBy('visitors.descr')
the result list is correct as only each visitor is shown once. But I need the latest occurrence of a visitor and the result list shows the first occurrence. So instead of ID 2 I would like to see ID 7.
The following query solves that issue:
select *
from visitors as v
INNER JOIN ( select descr , MAX(created_at) as max_created
from visitors as v
group by descr ) AS M
where v.descr = M.descr AND v.created_at = M.max_created
ORDER BY created_at DESC
Could someone help get this into eloquent or get me into another direction?

Answer 1

If you can use the AUTO_INCREMENT id column instead of created_at, you can use another way to get the (same) result:
select *
from visitors
where id in (
select MAX(id)
from visitors
group by descr
)
ORDER BY id DESC
This query is probably less efficient, but it's easier to use with eloquent, since whereIn() supports subqueries.
$subQuery = Visitor::groupBy('descr')->select(DB::raw('max(id)'));
$latestVisitors = Visitor::whereIn('id', $subQuery)->orderBy('id', 'desc')->get();
Note that your original query can return multiple rows per descr, if the value in created_at is the same. This can't happen using id since it is unique.

Answer 2

Why dont you just query the sql directly like this:
DB::table('visitors')->select("
select *
from visitors as v
INNER JOIN ( select descr , MAX(created_at) as max_created
from visitors as v
group by descr ) AS M
where v.descr = M.descr AND v.created_at = M.max_created
ORDER BY created_at DESC
");
In eloquent the query would be something like this:
DB::table('visitors as v')
->join(DB::raw("( select descr , MAX(created_at) as max_created
from visitors as v
group by descr ) AS M "),'v.id','=','M.id')
->select('*')
->where('v.descr','M.descr')->where('v.created_at','M.max_created')
->orderBy('created_at','desc')
->get()

Answer 3

You can write a helper relationship in your Visitor model
public function lastVisit()
{
$this->hasOne('VisitModel')->latest();
}
and then just fetch all visitors with their last visit
$visitors = Visitor::with('lastVisit')->get();
That's really all there is to it.