SELECT 
utl_raw.cast_to_varchar2(dbms_lob.substr(t.dync_xmlstr)) 
from t_wf_dcasedispatchmsg t 

dbms_lob.substr函数用来操作数据库的大型对象,叫做大型对象定位器 
utl_raw.cast_to_varchar2函数用来将blob转为varchar2 
长度限制:2000

如果长度超出2000

SELECT utl_raw.cast_to_varchar2(DBMS_LOB.SUBSTR(t.command,2000,1)),
utl_raw.cast_to_varchar2(DBMS_LOB.SUBSTR(t.command,2000,2001))
from nc_his.ESB_INTERFACE_LOG_H t