我可以在变量中存储选定的值

Is it possible to store an select value into an variable? in codeigniter. Something like this:

   $this->db->select_max('Datum');
        $this->db->from('Uren');
        $this->db->join('Project','Project.idProject = Uren.idProject');

 $test123 = $this->db->get();

So that i get the value of the select_max into the variable $test123

I think you need to do something like this.

$this->db->select_max('Uren.Datum');
$test = $this->db->get('table_name');

if ($test) {
    $this->db->select('fields');
    $this->db->group_by('Project.idProject');
    $this->db->get('table_name');
}

these will be two separate quires.

try

$this->db->select_max('field_name');
$check_query = $this->db->get('table_name');
if ($check_query->num_rows() > 0) {
$max = $check_query->result();
if(!empty($max[0]->field_name)) {
}
}

for more :- https://www.codeigniter.com/userguide2/database/active_record.html

The proprer way, then $max will contain the max of your request, or FALSE if request is not valid :

$query = $this->db->select_max('Uren.Datum', 'max_Datum')->get('your_table');      
$max = $query ? $query->row()->max_Datum : FALSE;

This is the answer that worked to my question. Hope it will help someone else too

    $this->db->select_max('Datum');
                        $this->db->from('Uren');
        $this->db->join('Project','Project.idProject = Uren.idProject');
        if ($idKlant > 0){
        $this->db->where('idKlant', $idKlant);}
            $datumstart = $this->db->get();
foreach ($datumstart->result() as $row)
{
    $datastart = $row->Datum;
}