Попробуйте это:
DB::table('demand_costings')
->select([
'pr_number as Demand_Costing',
'work_types',
'SUM(pr_quantity_in_pcs) as Total'
])
->join('work_types', 'demand_costings.worktype_id', '=', 'work_types.id')
->groupBy('work_types.work_types')
->get();