mysql裡面的while執行機制是先執行再判斷，當監聽語句判斷tag=1的時候，while裡面的語句會繼續執行，但是監聽語句裡面的tag=1會讓fetch語句失效，進而接著執行1select uid語句,所以會輸出兩次最後一個

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 do
fetch allinfo into uid;
select uid;
end while;
close allinfo;

end

輸出的資料中，最後兩個資料是相同的

只要在while迴圈體中加判斷就可以去除重複輸出的情況

eg：

create procedure get_all_info()
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 do
fetch allinfo into uid;
if tag=0 then
select uid;
end if;
end while;
close allinfo;


end