僅借鑑學習
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檯。
--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;
--創建結束標誌變量
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;
--開始循環遊標裏的數據
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,這裏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了,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預定義的NOT FOUND錯誤,所以在上面使用下面的代碼指定了當引發not found錯誤時定義一個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;
所以在循環時加上了下面這句代碼:
--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的事件只能定義一次,如果多定義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();
上面就是實現一個嵌套循環,當然這個例子比較牽強。湊合看看就行。
動態SQL
Mysql 支持動態SQL的功能,
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;