直接从varchar2 修改为clob会报错，不让修改，可以先修改为Long类型，然后从Long修改为clob .

而且更改为long，必须是 ORA-01439: 要修改的資料欄必須是空的, 才可以變更資料庫

You can use the ALTER TABLE statement in SQL to convert a LONG column to a LOB column. To do so, use the following syntax:

ALTER TABLE [.]
   MODIFY ( <long_column_name> { CLOB | BLOB | NCLOB } 
  [DEFAULT >]) [LOB_storage_clause];

For example, if you had a table that was created as follows:

CREATE TABLE Long_tab (id NUMBER, long_col LONG);

then you can change the column long_col in table Long_tab to datatype CLOB using following ALTER TABLE statement:

ALTER TABLE Long_tab MODIFY ( long_col CLOB );

ITPub博客

如何将varchar2修改为clob类型字段(使用long过渡)