[求助]select a into :i_pd from dual where exists (select * from yd_yprkb whe

我的PB程序如下：
string v_ypbm
int i_pd
v_ypbm='1111111111'

我建立了一个表dual ，a为数值型 1
select a into :i_pd from dual
where exists (select * from yd_yprkb where ypbm=:v_ypbm);

用以上方法i_pd的值总是为0

但改用下面语句可以得到i_pd的返回值为1

select a into :i_pd from dual
where exists (select * from yd_yprkb where ypbm='1111111111');

注：表yd_yprkb中 ypbm 字段为字符型

为什么把变量v_ypbm换成'1111111111'，就有返回值了呢，请高手指教！！！