什么时候会用到Oracle游标,以及其中的for、loop、if呢?
一、需求
什么时候会用到Oracle游标,以及其中的for、loop、if呢?
先看这样一个需求:
有一张学生授课表T_TEACHING,每个学生都有数门课程:
但是因为某些原因,导致有的学生课程不全(本应该每个学生都有3门课),应该如何把不全的学生检索出来,再给这些学生添加课程呢,并且要求能够快速解决这个问题。
二、分析
我们对需求进行一步步梳理:
1、我们不知道哪个学生课程不全,所以需要循环判断每个学生
通常用for循环:for USER_ID in USER_IDS
2、循环判断每个学生就要拿到所有的学号
select:select USER_ID from T_TEACHING GROUP BY USER_ID;
3、有了学号,判断这个学生是否缺少课程,然后添加这门课
(1)select:select count(*) from T_TEACHING where USER_ID=@USER_ID and COURSE_ID=@COURSE_ID;
(2)if else 判断
(3)insert:insert into T_TEACHING(COURSE_ID,USER_ID) values (@COURSE_ID,@USER_ID)
通过分析大致的流程出来了,然后再看细节:
我们通过select ..GROUP BY获取了授课表中所有的学号,然后就对这些学号进行循环获取,,关键在于select ..GROUP BY出来如何循环获取。
三、方法
在oracle中提供了“游标”
游标是什么,游标用来处理从数据库中检索的多行记录(使用SELECT语句)。利用游标,程序可以逐个地处理和遍历一次检索返回的整个记录集。
这正是我们想要的!!
结合for循环,我们可以使用for循环游标,格式如下:
--(1)定义游标
--关键字cursor
cursor x is select语句
--(2)定义游标变量
y x%rowtype
--(3)使用for循环来使用这个游标
for y in x loop
--逻辑处理
end loop;
下面我们通过sql存储过程来完整的实现上面的需求
declare
--定义类型
cursor t_tea
is
select USER_ID from T_TEACHING GROUP BY USER_ID;
--定义一个游标变量
t_row t_tea%rowtype;
--定义一个number类型的临时变量
v_count number;
begin
for t_row in t_tea loop
select count(*) into v_count from T_TEACHING where USER_ID=t_row.USER_ID and COURSE_ID=02;
if v_count = 0 then
insert into T_TEACHING(COURSE_ID,USER_ID) values (02,t_row.USER_ID);
end if;
end loop;
end;
四、总结
Oracle中的游标循环不只有for循环,Oracle中提供了两种游标,显示游标和隐式游标,其他游标的使用方式可以在这里了解,用到了在深入研究:

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