如何把varchar2类型的字段切换成clob

具体操作如下：

原字段bookdetails是varchar2(4000),原表已经有业务数据

ALTER TABLE test
ADD (bookdetails01 CLOB);

UPDATE test SET bookdetails01=bookdetails;
COMMIT;

ALTER TABLE test DROP COLUMN bookdetails;

ALTER TABLE atable
RENAME COLUMN bookdetails01 TO bookdetails;