MySQL 游标的定义与使用方式
创建游标
首先在MySql中创建一张数据表:
CREATE TABLE IF NOT EXISTS `store` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) NOT NULL,
`count` int(11) NOT NULL DEFAULT ‘1’,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=7;
INSERT INTO `store` (`id`, `name`, `count`) VALUES
(1, ‘android’, 15),
(2, ‘iphone’, 14),
(3, ‘iphone’, 20),
(4, ‘android’, 5),
(5, ‘android’, 13),
(6, ‘iphone’, 13);我们现在要用存储过程做一个功能,统计iphone的总库存是多少,并把总数输出到控制台。
–在windows系统中写存储过程时,如果需要使用declare声明变量,需要添加这个关键字,否则会报错。
delimiter //
drop procedure if exists StatisticStore;
CREATE PROCEDURE StatisticStore()
BEGIN
–创建接收游标数据的变量
declare c int;
declare n varchar(20);
–创建总数变量
declare total int default 0;
–创建结束标志变量
declare done int default false;
–创建游标
declare cur cursor for select name,count from store where name = ‘iphone’;
–指定游标循环结束时的返回值
declare continue HANDLER for not found set done = true;
–设置初始值
set total = 0;
–打开游标
open cur;
–开始循环游标里的数据
read_loop:loop
–根据游标当前指向的一条数据
fetch cur into n,c;
–判断游标的循环是否结束
if done then
leave read_loop; –跳出游标循环
end if;
–获取一条数据时,将count值进行累加操作,这里可以做任意你想做的操作,
set total = total + c;
–结束游标循环
end loop;
–关闭游标
close cur;
–输出结果
select total;
END;
–调用存储过程
call StatisticStore();fetch是获取游标当前指向的数据行,并将指针指向下一行,当游标已经指向最后一行时继续执行会造成游标溢出。
使用loop循环游标时,他本身是不会监控是否到最后一条数据了,像下面代码这种写法,就会造成死循环;
read_loop:loop
fetch cur into n,c;
set total = total+c;
end loop;在MySql中,造成游标溢出时会引发mysql预定义的NOT FOUND错误,所以在上面使用下面的代码指定了当引发not found错误时定义一个continue 的事件,指定这个事件发生时修改done变量的值。
declare continue HANDLER for not found set done = true;所以在循环时加上了下面这句代码:
–判断游标的循环是否结束
if done then
leave read_loop; –跳出游标循环
end if;如果done的值是true,就结束循环。继续执行下面的代码。
使用方式
游标有三种使用方式:
第一种就是上面的实现,使用loop循环;
第二种方式如下,使用while循环:
drop procedure if exists StatisticStore1;
CREATE PROCEDURE StatisticStore1()
BEGIN
declare c int;
declare n varchar(20);
declare total int default 0;
declare done int default false;
declare cur cursor for select name,count from store where name = ‘iphone’;
declare continue HANDLER for not found set done = true;
set total = 0;
open cur;
fetch cur into n,c;
while(not done) do
set total = total + c;
fetch cur into n,c;
end while;
close cur;
select total;
END;
call StatisticStore1();第三种方式是使用repeat执行:
drop procedure if exists StatisticStore2;
CREATE PROCEDURE StatisticStore2()
BEGIN
declare c int;
declare n varchar(20);
declare total int default 0;
declare done int default false;
declare cur cursor for select name,count from store where name = ‘iphone’;
declare continue HANDLER for not found set done = true;
set total = 0;
open cur;
repeat
fetch cur into n,c;
if not done then
set total = total + c;
end if;
until done end repeat;
close cur;
select total;
END;
call StatisticStore2();
游标嵌套
在mysql中,每个begin end 块都是一个独立的scope区域,由于MySql中同一个error的事件只能定义一次,如果多定义的话在编译时会提示Duplicate handler declared in the same block。
drop procedure if exists StatisticStore3;
CREATE PROCEDURE StatisticStore3()
BEGIN
declare _n varchar(20);
declare done int default false;
declare cur cursor for select name from store group by name;
declare continue HANDLER for not found set done = true;
open cur;
read_loop:loop
fetch cur into _n;
if done then
leave read_loop;
end if;
begin
declare c int;
declare n varchar(20);
declare total int default 0;
declare done int default false;
declare cur cursor for select name,count from store where name = ‘iphone’;
declare continue HANDLER for not found set done = true;
set total = 0;
open cur;
iphone_loop:loop
fetch cur into n,c;
if done then
leave iphone_loop;
end if;
set total = total + c;
end loop;
close cur;
select _n,n,total;
end;
begin
declare c int;
declare n varchar(20);
declare total int default 0;
declare done int default false;
declare cur cursor for select name,count from store where name = ‘android’;
declare continue HANDLER for not found set done = true;
set total = 0;
open cur;
android_loop:loop
fetch cur into n,c;
if done then
leave android_loop;
end if;
set total = total + c;
end loop;
close cur;
select _n,n,total;
end;
begin
end;
end loop;
close cur;
END;
call StatisticStore3();上面就是实现一个嵌套循环,当然这个例子比较牵强。凑合看看就行。。
动态SQL
Mysql 支持动态SQL的功能,
set @sqlStr=’select * from table where condition1 = ?’;
prepare s1 for @sqlStr;
–如果有多个参数用逗号分隔
execute s1 using @condition1;
–手工释放,或者是 connection 关闭时, server 自动回收
deallocate prepare s1;以上就是MySQL 游标的定义与使用方式的详细内容,更多关于MySQL 游标的资料请关注共生网络其它相关文章!
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