先用to_clob转换，to_clob转换long类型只能用于insert语句中

CREATE TABLE new_table (col1, col2, ... lob_col CLOB);

INSERT INTO new_table 
  select o.col1, o.col2, ...
        TO_LOB(o.old_long_col
   FROM old_table o;

然后在对新表内容进行查询即可

参考
https://docs.oracle.com/cd/E11882_01/server.112/e41084/functions205.htm#SQLRF06134