oracle - 计算表中的记录并使用游标更新表

Question

我在 Oracle 中有以下表:

Teacher
id_teacher (pk)
number_courses

Course
id_course (pk)
id_teacher (fk)

我想创建一个游标，通过计算教师分配的类(class)来更新 Teacher 表中的 number_courses 字段。据我所知，我应该首先声明一个这样的游标:

cursor c_teacher IS
        select id_teacher from teacher;

然后执行一个 for 循环，迭代此光标的结果并计算分配的类(class)，我的解决方案草案是:

declare
  countC number(2);
    cursor c_teacher IS
        select id_teacher from teacher;
begin
    for data in c_teacher
    loop
        select count(id_teacher) into countC from Course where id_teacher=data;
        --I can output here with a DMBS_OUTPUT only to see if its working, but
        --I need to use an UPDATE instruction
    end loop;
end;

Answer 1

不要使用光标。 (除非这是您面临的实际问题的极大简化。或者这是一个考虑不周的家庭作业问题。在这种情况下， secret 是 update where current of )

带有相关子查询的单个更新语句即可完成这项工作:

update Teachers T
set number_courses = (select count(*)
    from Courses C
    where C.id_teacher = T.id_teacher);

由于值不会不同步，可能更好的是不在教师表中存储类(class)数量并在需要时计算正确的值:

alter table Teachers drop column number_courses;

create view Teachers_VW as
select T.id_teacher
    , count(*) as number_courses
from Teachers T
left outer join Courses C on C.id_teacher = T.id_teacher
group by T.id_teacher;