MySql中遊標的設定與三種使用方式。
創建遊標
首先在MySql中創建一張數據表：
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);
我們現在要用存儲過程做一個功能，統計iphone的總庫存是多少，並把總數輸出到控制臺。
–在windows系統中寫存儲過程時，如果需要使用declare聲明變量，需要添加這個關鍵字，否則會報錯。
delimiter //
drop procedure if exists StatisticStore;
CREATE PROCEDURE StatisticStore()
BEGIN
–創建接收遊標數據的變量
declare c int;
declare n varchar(20);
–創建總數變量
declare total int default 0;
–創建結束標志變量
declare done int default false;
–創建遊標
declare cur cursor for select name,count from store where name = 'iphone';
–指定遊標循環結束時的返回值
declare continue HANDLER for not found set done = true;
–設置初始值
set total = 0;
–打開遊標
open cur;
–開始循環遊標裡的數據
read_loop:loop
–根據遊標當前指向的一條數據
fetch cur into n,c;
–判斷遊標的循環是否結束
if done then
leave read_loop; –跳出遊標循環
end if;
–獲取一條數據時，將count值進行累加操作，這裡可以做任意你想做的操作，
set total = total + c;
–結束遊標循環
end loop;
–關閉遊標
close cur;
–輸出結果
select total;
END;
–調用存儲過程
call StatisticStore();
fetch是獲取遊標當前指向的數據行，並將指針指向下一行，當遊標已經指向最後一行時繼續執行會造成遊標溢出。
使用loop循環遊標時，他本身是不會監控是否到最後一條數據瞭，像下面代碼這種寫法，就會造成死循環;
read_loop:loop
fetch cur into n,c;
set total = total+c;
end loop;
在MySql中，造成遊標溢出時會引發mysql預設定的NOT FOUND錯誤，所以在上面使用下面的代碼指定瞭當引發not found錯誤時設定一個continue 的事件，指定這個事件發生時修改done變量的值。
declare continue HANDLER for not found set done = true;[sql] view plain copy
–判斷遊標的循環是否結束
if done then
leave read_loop; –跳出遊標循環
end if;
如果done的值是true，就結束循環。繼續執行下面的代碼。
使用方式
遊標有三種使用方式：
第一種就是上面的實現，使用loop循環;
第二種方式如下，使用while循環：
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();
第三種方式是使用repeat執行：
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();
遊標嵌套
在mysql中，每個begin end 塊都是一個獨立的scope區域，由於MySql中同一個error的事件隻能設定一次，如果多設定的話在編譯時會提示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();[sql] view plain copy
set @sqlStr='select * from table where condition1 = ?';
prepare s1 for @sqlStr;
–如果有多個參數用逗號分隔
execute s1 using @condition1;
–手工釋放，或者是 connection 關閉時， server 自動回收
deallocate prepare s1;
