ORA-00932: 不一致的数据类型: 要求 - 得到的却是 BLOB

执行如下sql会报错：
ORA-00932: 不一致的数据类型: 要求 - 得到的却是 BLOB


select distinct (docs.id),
                docs.title,
                docs.documentNo,
                docs.dispatchState,
                docs.dispatchDate,
                docs.dispatchUnit,
                docs.theme,
                docs.secretDegree,
               docs.contents,                                 
                docs.urgentDegree,
                docs.synchroDocs,
                docs.attachFiles,
                docs.fullPinyin,
                docs.simplePinyin,
                docs.createUser,
                docs.updateUser,
                docs.updateDate,
                docs.createDate,
                docs.signstate
  from (select doc.*, docHasOrg.Signstate
          from documents doc, documentsHasOrg docHasOrg
         where doc.id = docHasOrg.documentId) docs

原因是：
blob字段不允许distinct、insert、update等操作。他是特殊的对象，其实数据库里纪录的只是一个类似指针的对象，就象一个视图的概念

表结构如下：