奇怪的查询行为oracle sql - 将文本转换为数字

Question

oracle query这是有效的

select *
from (
select to_number(substr(app_cluster,6,2), '99') as  b
from xtern_app_info
WHERE app_cluster IS NOT NULL
AND APP_CLUSTER <> 'CLUSTER'
);

但在添加＆＃39;其中b＆gt; 2＆＃39;出错了，为什么？

select *
from (
select to_number(substr(app_cluster,6,2), '99') as  b
from xtern_app_info
WHERE app_cluster IS NOT NULL
AND APP_CLUSTER <> 'CLUSTER'
) where b > 2;

ORA-29913：执行ODCIEXTTABLEFETCH标注时出错 ORA-01722：无效的号码 29913. 00000 - ＆＃34;执行％s标注错误＆＃34; *原因：执行指定的标注导致错误。 *操作：检查错误消息是否采取适当的措施。