oracle varchar 改为 clob

下面的例子是把comments列的类型由varchar2改为clob

alter table mytableName  rename  column   comments  to  commentsback;
alter  table  mytableName   add  comments  long;
update  mytableName     set  comments= commentsback ;
commit;
alter  table  mytableName     modify(comments  clob);
alter table mytableName   drop  column   commentsback;