--修改dishdetail字段名dishdetailback
ALTER TABLE dispatch_dish RENAME COLUMN dishdetail TO dishdetailback;
--创建一个dishdetail字段  类型是clob
ALTER TABLE dispatch_dish ADD dishdetail  clob;
--把dishdetailback的值都放到dishdetail中
UPDATE dispatch_dish SET dishdetail =dishdetailback  -- CAST(dishdetailback AS clob);
--删除dishdetailback字段
ALTER TABLE dispatch_dish DROP COLUMN dishdetailback;

 

 

要把一个varchar2的字段修改成clob类型，前提是已存在的数据不能够改变。