oracle - 当我使用表中的一些值更新同一个表时，给出子查询返回太多行错误

Question

以下查询返回

ORA-01427: 单行子查询返回多于一行

UPDATE master 
   SET final = NVL(final,
              (SELECT 'Disconnect' 
                 FROM  (SELECT NVL(Ported_out_flag, type) as type, 
                               CASE WHEN order = 'C'  THEN 'C' ELSE service end service,
                               cdms
                          FROM master) 
                WHERE type in ('PORTED_OUT')
                  AND (service in ('C','I') or service is null)
                  AND cdms in ('FR','IS','RE','QU')
              )
             );

有人能帮我一下吗？