大家好，我用mysql写存储过程自动产生编号，运行之后编号只有1，请问哪儿有问题呢？

BEGIN
DECLARE subjectNo1 char(8);
DECLARE subjectNo2 char(8);
DECLARE nowYear char(4);
DECLARE Num DECIMAL;

select date_format(now(),'%Y') into nowYear from DUAL;
select ifnull(max(Subject_No),'0') into subjectNo1
from subject
where Subject_No like nowYear||'%';
if subjectNo1='0' then
set subjectNo2=nowYear||'0001';
else
set Num=to_number(substr(subjectNo1,5,4))+1;
if Num<10 then set subjectNo2=nowYear||'000'||ltrim(to_char(Num));
elseif Num<100 then set subjectNo2=nowYear||'00'||ltrim(to_char(Num));
elseif Num<1000 then set subjectNo2=nowYear||'0'||ltrim(to_char(Num));
else
set subjectNo2=nowYear||ltrim(to_char(Num));
end if;
end if;
set newSubjectNo=subjectNo2;
END