子句触发子句时的子查询函数

Question

如果学生的AVG（RESULT）超过3，我想创建一个阻止删除记录的触发器

  CREATE OR REPLACE TRIGGER DeleteStudentResults
  BEFORE DELETE ON SESSION_RESULTS
  REFERENCING OLD AS old NEW AS new
  FOR EACH ROW
  WHEN ((SELECT AVG(RESULT) from session_results where STUDENT_ID = :new.STUDENT_ID)>= 3)
  BEGIN
   raise_application_error (-20100, 'You can not delete initial record');
 END;

但甲骨文表示禁止在子句中包含SELECT语句。 如果平均值大于或等于3，我怎样才能实现阻止删除的行为？

Answer 1

将其视为建议

  CREATE OR REPLACE TRIGGER DeleteStudentResults
  BEFORE DELETE ON SESSION_RESULTS
  REFERENCING OLD AS old NEW AS new
  FOR EACH ROW
  declare 
  avg_result number;
  BEGIN
  (SELECT AVG(RESULT) into avg_result from session_results where STUDENT_ID = :old.STUDENT_ID);
  if(avg_result>=3) then
   raise_application_error (-20100, 'You can not delete initial record');
   end if;
 END;

Answer 2

通过删除触发器

来解决这个问题并不是一个好主意

例如使用session_results

id   studend_id   result
1    9            1
5    9            2
7    9            5

如果删除此学生，则取决于行的处理顺序。

 delete id=1  avg(1,2,5)= 2.7
 delete id=5  avg(2,5)=3.5 --> raise error

但

 delete id=7  avg(1,2,5)= 2.7
 delete id=1  avg(1,2)=1.5 
 delete id=5  avg(2)=2

尝试通过物化视图解决此问题，有关详细信息，请参阅http://www.oracle.com/technetwork/testcontent/o58asktom-101055.html