如何判断oracle大字段（clob）为空？
http://topic.csdn.net/u/20081110/15/094fcaf8-8b93-42c1-b8a9-364dc27f3e55.html
--assume x is a clob field
--x is null
--x equal to empth_clob, dbms_lob.getlength(x) = 0
select * from a where x is null or dbms_lob.getlength(x) = 0
DBMS_LOB为专门处理大对象的一个包
DBMS_LOB.GETLENGTH是查大对象数据类型的大小