oracle clob与nclob的互相转换

 drop table clobTets
 create table clobTets(
 col1 nclob
 )
 select * from clobTets
 insert into clobTets values('11111')
 alter table clobTets add (col2 varchar2(4000))
 update clobTets set col2 = dbms_lob.substr(col1,4000)
 alter table clobTets drop column col1
 alter table clobTets rename column col2 to col1; 
 
 alter table clobTets add (col2 clob)
 update clobTets set col2 = col1
 alter table clobTets drop column col1
 alter table clobTets rename column col2 to col1;