MySQL游标？ - Deep-Web/MarkDownPhotos GitHub Wiki

创建游标

首先在MySql中创建一张数据表：

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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。

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–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；

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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。

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declare continue HANDLER for not found set done = true;

所以在循环时加上了下面这句代码：

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–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：

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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：

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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。

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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，

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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;

MySQL游标？ - Deep-Web/MarkDownPhotos GitHub Wiki

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