sql - 如何在oracle中将触发器和光标与for循环结合使用？

Question

假设有两个表

 1.student(roll_no, class, credits)
 2.class(class, total_student, Total_credits)

如何使用学生表上的光标创建触发器，该触发器将使用学生总数及其总学分更新班级？在每次插入或删除时

Answer 1

表的更新total_student和total_credit列将涉及在表上定义的触发器中class针对表编写查询。这样做会导致ORA-04091: table name is mutating, trigger/function may not see it错误。为避免该错误，至少有三种方法可以在 每次表中发生更改（删除/插入/更新）时获取和更新。假设 master( )/detail( ) 关系中的表：studentstudenttotal_studenttotal_credits studentclassstudent

1. 第一种方法（最大的）将涉及创建几个数据库对象：

   1. 嵌套表 SQL 类型

      create or replace type T_clasids is table of number;
      

   2. 将具有该 SQL 类型的变量来存储类IDs 的包。不知何故受到了 DML 语句的影响。

      包装规格：

      create or replace package Pkg
      is
        procedure reset_list_of_ids;      -- empties out the list of class ids
        procedure add_id(p_id in number); -- add new class id to a list if row
                                          -- with this ID has been affected by  
                                          -- a DML statement
        procedure update_class;           -- updates class table
      end;
      

      包体：

      create or replace package body PKG
      is
      
         g_classids T_clasids := T_clasids();
      
      procedure reset_list_of_ids
      is
      begin
        g_classids.delete;
      end;
      
      procedure add_id(p_id in number)
      is
      begin
        g_classids.extend;
        g_classids(g_classids.count) := p_id;
      end;
      
      procedure update_class
      is
      begin
        update class t
           set (  t.total_student
                , t.total_credits ) = ( select count(*)
                                             , sum(s.credits)
                                          from student s
                                         where s.class = t.class)
         where t.class in (select column_value
                             from table(g_classids));
      
       end;
      

      结尾;

   3. 三个触发器： a) Before 语句；b) 排后；c) 声明之后。

      -- before insert/update/delete statement level trigger 
      -- to empty out the class id list
      create or replace trigger tr_bs_initialize
      before insert or delete or update on student
      begin
        pkg.reset_list_of_ids;
      end;
      
      
      
      -- after insert/update/delete statement level trigger
      -- to update class table with new information 
      create or replace trigger tr_as_update_class
      after insert or delete or update on student
      begin
        pkg.update_class;
      end;
      
      
      -- after insert/update/delete row level trigger
      -- to populate class id collection with ids of 
      -- rows which has been affected by a DML statement
      create or replace trigger tr_ar_populate
      after insert or delete or update on student
      for each row
      begin
         -- nvl(:new.class, :old.class)
         -- in a case :new.clas happens to be null
         pkg.add_id(nvl(:new.class, :old.class));
      end;
      

      这是它如何工作的示例：

      select t.* from class t;
      
      CLASS      TOTAL_STUDENT TOTAL_CREDITS
      ---------- ------------- -------------
       1         null          null     
       2         null          null 
       3         null          null
      
       insert into student(roll_no, class, credits)
          values(1, 2, 3);
      
      
       select t.* from class t;
      
       CLASS      TOTAL_STUDENT TOTAL_CREDITS
       ---------- ------------- -------------
       1             null          null 
       2             1             3
       3             null          null
      

2. 第二种方法（最短的一种和个人首选的一种）是从表中删除total_student并创建一个视图，该视图将计算并保持有关班级学生总数和学分总和的最新信息：total_creditsclass

       create or replace view v_class as
          select c.class
               , count(s.class) as total_students
               , sum(s.credits) as total_credits
            from student s
           right join class c
              on (c.class = s.class)
           group by c.class
   
        select t.* from v_class t;
   
        CLASS      TOTAL_STUDENTS TOTAL_CREDITS
        ---------- -------------  -------------
        1             null          null 
        2             1             3
        3             null          null
   

3. 第三种方法。在表上定义一个插入/更新/删除语句级触发器sudent并使用merge语句更新class表：

        create or replace trigger tr_aiudsl
        after insert or update or delete on student
        begin
           merge into class c
           using (select t.class
                       , count(*)      as total_students
                       , sum(t.credits)as total_credit
                    from student t
                   group by t.class) q
                      on (q.class = c.class)
            when matched
            then update
              set c.total_student = q.total_students
                , c.total_credits  = q.total_credit;
        end;
   

   了解有关merge声明的更多信息。