子查询在如果不指定表的情况下默认在自己表中匹配，条件字段找不到会报无此字段，若存在会进行全表扫描

--正确
UPDATE WEB_PLY_BASE B
   SET B.N_AMT_VAR =
       (SELECT SUM(A.N_AMT_VAR)
          FROM WEB_PLY_CVRG A
         WHERE A.C_APP_NO = B.C_APP_NO)
 WHERE B.C_PLY_NO = '199320606022016000064';
--错误
UPDATE WEB_PLY_BASE
   SET N_AMT_VAR =
       (SELECT SUM(A.N_AMT_VAR)
          FROM WEB_PLY_CVRG A
         WHERE A.C_APP_NO = C_APP_NO)
 WHERE C_PLY_NO = '199320606022016000064';