缺少关键字（ORA-00905） - Oracle SQL案例陈述？

Question

你好任何人都可以帮助我在使用案例陈述时遇到这些错误

(select distinct case 

     when mtt.transaction_type_name   = 'Miscellaneous Receipt'  then pha.segment1 is  null
     when mtt.transaction_type_name  = 'Return from Maintenance' then pha.segment1 is  null
     else mtt.transaction_type_name
      END AS trans
     from  mtl_material_transactions mmt                             
     where 1=1 
     AND msi.inventory_item_id = mmt.inventory_item_id
     AND ph.po_header_id = pha.po_header_id

    )