calculating with values from two different mysql tables - mysql

Question

I'm building a sort of human resources application where I have to reduce a variable of a mysql table with a variable of another mysql table.
The CMV I'm using is Laravel so I'm trying to use the DB class as much as possible.
This is the first table, users:
users_table
And the second table, aanvraags:
aanvraags_table
I would like reduce the variable vakantie from the first table with the variable duur from the second table. This would happen right after the vacation request is accepted. There is already a function in my controller to change the variable goedgekeurd from 0 to 1. The function looks like this:
public function updateNieuw($id){
//update aanvraag
$update_aanvraag = DB::table('aanvraags')
->where('id', $id)
->update([
'goedgekeurd' => 1 //approved by the boss
]);
Within that same function I would like to reduce the variable vakantie with the variable duur.
At the moment I already tried a few things like:
$update_vakantiedag = DB::table('aanvraags')
->select('duur')
->where('id', $id)
->get();
$vakantie = DB::table('users')
->join('aanvraags', 'users.id', '=', 'aanvraags.user_id')
->where('aanvraags.id', '=', $id)
->update([
'users.vakantie' => $update_vakantiedag
]);
All of this doesn't get me anywhere. Most of the time I get errors or the value from the variable vakantie from the first table.
It is quite important for the application that the reduction takes place. The variable vakantie displays the remaining vacation days per user after accepting a new request and is displayed in the app.
Any tips or solutions are welcome !
Thank you all in advance !

Answer 1

Use this:
$update_vakantiedag = DB::table('aanvraags')
->select('duur')
->where('id', $id)
->first();
$vakantie = DB::table('users')
->join('aanvraags', 'users.id', '=', 'aanvraags.user_id')
->where('aanvraags.id', '=', $id)
->decrement('users.vakantie', $update_vakantiedag->duur);

Answer 2

#JonasStaudenmeir Thanks a lot ! I changed the function and it works !
$update_vakantiedag = DB::table('aanvraags')
->select('duur')
->where('id', $id)
->first();
$verlof = DB::table('users')
->join('aanvraags', 'users.id', '=', 'aanvraags.user_id')
->where('aanvraags.id', '=', $id)
->first();`
$vakantie = DB::table('users')
->join('aanvraags', 'users.id', '=', 'aanvraags.user_id')
->where('aanvraags.id', '=', $id)
->update([
'users.vakantie' => $verlof->vakantie - $update_vakantiedag->duur
]);
Thank you so much !