首先在MySql中創(chuàng)建一張數(shù)據(jù)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(xiàn)在要用存儲過程做一個功能,統(tǒng)計iphone的總庫存是多少,并把總數(shù)輸出到控制臺。
--在windows系統(tǒng)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(guān)鍵字,否則會報錯。 delimiter // drop procedure if exists StatisticStore; CREATE PROCEDURE StatisticStore() BEGIN --創(chuàng)建接收游標數(shù)據(jù)的變量 declare c int; declare n varchar(20); --創(chuàng)建總數(shù)變量 declare total int default 0; --創(chuàng)建結(jié)束標志變量 declare done int default false; --創(chuàng)建游標 declare cur cursor for select name,count from store where name = 'iphone'; --指定游標循環(huán)結(jié)束時的返回值 declare continue HANDLER for not found set done = true; --設(shè)置初始值 set total = 0; --打開游標 open cur; --開始循環(huán)游標里的數(shù)據(jù) read_loop:loop --根據(jù)游標當前指向的一條數(shù)據(jù) fetch cur into n,c; --判斷游標的循環(huán)是否結(jié)束 if done then leave read_loop; --跳出游標循環(huán) end if; --獲取一條數(shù)據(jù)時,將count值進行累加操作,這里可以做任意你想做的操作, set total = total + c; --結(jié)束游標循環(huán) end loop; --關(guān)閉游標 close cur; --輸出結(jié)果 select total; END; --調(diào)用存儲過程 call StatisticStore();
fetch是獲取游標當前指向的數(shù)據(jù)行,并將指針指向下一行,當游標已經(jīng)指向最后一行時繼續(xù)執(zhí)行會造成游標溢出。
使用loop循環(huán)游標時,他本身是不會監(jiān)控是否到最后一條數(shù)據(jù)了,像下面代碼這種寫法,就會造成死循環(huán);
read_loop:loop fetch cur into n,c; set total = total+c; end loop;
在MySql中,造成游標溢出時會引發(fā)mysql預(yù)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(fā)not found錯誤時定義一個continue 的事件,指定這個事件發(fā)生時修改done變量的值。
declare continue HANDLER for not found set done = true;
所以在循環(huán)時加上了下面這句代碼:
--判斷游標的循環(huán)是否結(jié)束 if done then leave read_loop; --跳出游標循環(huán) end if;
如果done的值是true,就結(jié)束循環(huán)。繼續(xù)執(zhí)行下面的代碼。
游標有三種使用方式:
第一種就是上面的實現(xiàn),使用loop循環(huán);
第二種方式如下,使用while循環(huán):
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(zhí)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();
在mysql中,每個begin end 塊都是一個獨立的scope區(qū)域,由于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(xiàn)一個嵌套循環(huán),當然這個例子比較牽強。湊合看看就行。。
Mysql 支持動態(tài)SQL的功能,
set @sqlStr='select * from table where condition1 = ?'; prepare s1 for @sqlStr; --如果有多個參數(shù)用逗號分隔 execute s1 using @condition1; --手工釋放,或者是 connection 關(guān)閉時, server 自動回收 deallocate prepare s1;
以上就是MySQL 游標的定義與使用方式的詳細內(nèi)容,更多關(guān)于MySQL 游標的資料請關(guān)注腳本之家其它相關(guān)文章!