删除记录一旦删除了源表中ID的所有记录，就可以使用触发器

Question

我有两个桌子。由于我不能共享表名，因为它太具体了，所以我将尝试使用通用表解释我的查询。

这两个表是Score_table；哪个商店存储学生的分数，并且可以说另一个表是一个分数仪表板（Student_Score_Table），它提供了查看分数。

Score_table
|------------|------------|-------|
| Student ID | Subject ID | Score |  
|------------|------------|-------|
|     12     |      1     |   50  |
|------------|------------|-------|
|     12     |      2     |   70  |

Student_Score_Table
|--------------|------------|----------|
| Student Name | Subject A  | Subject B|  
|--------------|------------|----------|
|     Daniel   |      50    |    90    |
|--------------|------------|----------|
|     James    |      70    |   45     |

学生和科目都有各自的查找表，从中可以获取学生姓名和科目。

有一项服务可以更新和删除Score_table

中的记录

我在思考如何编写删除触发器时遇到问题，如何在此处编写触发器，该触发器将： 1.删​​除分数Student_Score_Table，即当我更新Score_table中的值时将其设置为null 2.从Student_Score_Table

中删除特定学生的所有记录后，删除Score_table中的整个记录​​。

我尝试在Score_table上编写一个触发后删除操作，并在使用计数查询删除Student_Score_Table之前检查学生是否有任何记录，但是我得到的结果为'1'而不是' 0'计数。

Answer 1

我将使用简单的数据透视查询来代替使用单独的表和触发器：

select *
  from (
    select subject_name, student_name, score
      from score 
      join students using (student_id)
      join subjects using (subject_id))
  pivot (max(score) for subject_name in ('Subject A', 'Subject B'))

^{dbfiddle demo}

创建视图，使用它代替表student_subject_score，而忽略触发器和与同步相关的问题。

您当前的解决方案使事情变得复杂。您可以在score上触发，但要检查特定学生是否存在任何行，也需要在表分数上select。而且行级触发器不允许这样做，从而导致mutating table错误。为了解决这个问题，您可能需要复合触发器。同样，您的设计需要复杂的case when（因为我们必须找到要无效的列）或动态SQL。旧的，简单而可靠的view能解决所有问题吗？

Answer 2

请参阅下面更新的触发代码。与上一个对此问题的最后答案相比，我应用于触发器的更新确实带有一些警告。请参阅THIS SO Post来了解这些问题：

触发代码

create or replace TRIGGER student_score_trig
  AFTER
   DELETE OR UPDATE of score
  ON score_table
 REFERENCING NEW AS NEW OLD AS OLD
  FOR EACH ROW

DECLARE
  /* The below pragma allows us to SELECT from the same table that Triggered the Trigger.
   | Many circles say that doing any 'work' on the same table as the Trigger
   | is dangerous due to possible uncommited changes that are not able to be
   | seen by the code within the Trigger.
  */
  pragma autonomous_transaction;

  n_student_count  NUMBER  :=  0;

 BEGIN

  IF UPDATING THEN
    /* Not sure how you plan to tie the Subjects between each of the two tables
     | but I'm guessing this would be a non-issue for you.
    */
    UPDATE student_score_table SET subject_a = NULL
    WHERE student_id = :old.student_id
    ;
    COMMIT;
    dbms_output.put_line('This student_id was UPDATED: ' || :old.student_id || ' for subject_id: ' || :old.subject_id);
  END IF;


  IF DELETING THEN

    SELECT COUNT(student_id)
    INTO n_student_count
    FROM score_table
    WHERE student_id = :old.student_id
    ;

    dbms_output.put_line(n_student_count);

    IF n_student_count <= 1 THEN
      DELETE student_score_table
      WHERE student_id = :old.student_id
      ;
      COMMIT;
      dbms_output.put_line('This student_id was DELETED: ' || :old.student_id);
    END IF;

  END IF;

 exception
   when others then
     raise;
END;

  

下面的完整设置 ...创建表...插入数据和编译触发器都在下面：

CREATE TABLE score_table 
( student_id  NUMBER,
  subject_id  NUMBER,
  score       NUMBER  
)
;
/

INSERT INTO score_table VALUES (12, 1, 50);
INSERT INTO score_table VALUES (12, 2, 70);
INSERT INTO score_table VALUES (10, 5, 60);

/

CREATE TABLE student_score_table 
( student_id    NUMBER,
  student_name  VARCHAR2(250),
  subject_a     NUMBER,
  subject_b     NUMBER
)
;

INSERT INTO student_score_table VALUES (12, 'Daniel', 50, 90);
INSERT INTO student_score_table VALUES (10, 'James', 70, 45);

/

--set define off; /* Depending on your interface this line might need to be removed */
--/

create or replace TRIGGER student_score_trig
  AFTER
   DELETE OR UPDATE of score
  ON score_table
 REFERENCING NEW AS NEW OLD AS OLD
  FOR EACH ROW

DECLARE
  /* The below pragma allows us to SELECT from the same table that Triggered the Trigger.
   | Many circles say that doing any 'work' on the same table as the Trigger
   | is dangerous due to possible uncommited changes that are not able to be
   | seen by the code within the Trigger.
  */
  pragma autonomous_transaction;

  n_student_count  NUMBER  :=  0;

 BEGIN

  IF UPDATING THEN
    /* Not sure how you plan to tie the Subjects between each of the two tables
     | but I'm guessing this would be a non-issue for you.
    */
    UPDATE student_score_table SET subject_a = NULL
    WHERE student_id = :old.student_id
    ;
    COMMIT;
    dbms_output.put_line('This student_id was UPDATED: ' || :old.student_id || ' for subject_id: ' || :old.subject_id);
  END IF;


  IF DELETING THEN

    SELECT COUNT(student_id)
    INTO n_student_count
    FROM score_table
    WHERE student_id = :old.student_id
    ;

    dbms_output.put_line(n_student_count);

    IF n_student_count <= 1 THEN
      DELETE student_score_table
      WHERE student_id = :old.student_id
      ;
      COMMIT;
      dbms_output.put_line('This student_id was DELETED: ' || :old.student_id);
    END IF;

  END IF;

 exception
   when others then
     raise;
END;

  

查看实际操作

Answer 3

虽然不能百分百确定自己需要什么，但我可以使用此触发器完成我认为要实现的目标：

CREATE OR REPLACE TRIGGER student_score_trig
  AFTER
   DELETE OR UPDATE of score
  ON score_table
 REFERENCING NEW AS NEW OLD AS OLD
  FOR EACH ROW

 BEGIN

  DELETE student_score_table
  WHERE student_id = :old.student_id
  ;

 exception
   when others then
     raise;
END;

  

下面的完整设置...创建表...插入数据和编译触发器都在下面：

CREATE TABLE score_table 
( student_id  NUMBER,
  subject_id  NUMBER,
  score       NUMBER  
)
;
/

INSERT INTO score_table VALUES (12, 1, 50);
INSERT INTO score_table VALUES (12, 2, 70);
INSERT INTO score_table VALUES (10, 5, 60);

/

CREATE TABLE student_score_table 
( student_id    NUMBER,
  student_name  VARCHAR2(250),
  subject_a     NUMBER,
  subject_b     NUMBER
)
;

INSERT INTO student_score_table VALUES (12, 'Daniel', 50, 90);
INSERT INTO student_score_table VALUES (10, 'James', 70, 45);

/

set define off; /* Depending on your interface this line might need to be removed */

CREATE OR REPLACE TRIGGER student_score_trig
  AFTER
   DELETE OR UPDATE of score
  ON score_table
 REFERENCING NEW AS NEW OLD AS OLD
  FOR EACH ROW

 BEGIN

  DELETE student_score_table
  WHERE student_id = :old.student_id
  ;

 exception
   when others then
     raise;
END;

  

查看实际操作