mysql里面的while里面为什么会重复输出最后一个数据

mysql里面的while运行机制是先执行再判断，当监听语句判断tag=1的时候，while里面的语句会继续执行，但是监听语句里面的tag=1会让fetch语句失效，进而接着执行1select uid语句,所以会输出两次最后一个

eg:

create procedure get_all_info12()begin

declare uid int(5) default 0;declare tag int(1) default 0;declare allinfo cursor for select id from stu_info;declare continue handler for sqlstate '02000' set tag=1;

open allinfo;while tag=0 dofetch allinfo into uid;select uid;end while;close allinfo;

end

输出的数据中，最后两个数据是相同的

只要在while循环体中加判断就可以去除重复输出的情况

eg：

create procedure get_all_info()begindeclare uid int(5) default 0;declare tag int(1) default 0;declare allinfo cursor for select id from stu_info;declare continue handler for sqlstate '02000' set tag=1;open allinfo;while tag=0 dofetch allinfo into uid;if tag=0 thenselect uid;end if;end while;close allinfo;end