Illuminate\Database\Query\Builder::addSelect PHP Method

    public function addSelect($column)
    {
        $column = is_array($column) ? $column : func_get_args();
        $this->columns = array_merge((array) $this->columns, $column);
        return $this;
    }

 public function apply(QueryBuilder $builder)
 {
     if (!$this->request->has('_DatatableQuery') || !isset($this->filters->idField)) {
         $builder->addSelect($this->db->raw('0 as _checked'));
         return $builder;
     }
     $this->filters->items[] = -1;
     if ($this->filters->checkedAll) {
         $builder->addSelect($this->db->raw('(case when ' . $this->filters->idField . ' IN (' . implode(',', $this->filters->items) . ') then 0 else 1 end) as _checked'));
         return $builder;
     }
     $builder->addSelect($this->db->raw('(case when ' . $this->filters->idField . ' IN (' . implode(',', $this->filters->items) . ') then 1 else 0 end) as _checked'));
     return $builder;
 }