管理表的列

alter table .....modify

ALTER TABLE hr.admin_emp
      ADD (bonus NUMBER (7,2));
如果给basic compression 的table 添加列，则不能指定缺省值。
如果给oltp compression 的table 添加列，则可以添加缺省值，但是该列必须
非空（not null）

ALTER TABLE hr.admin_emp
      RENAME COLUMN comm TO commission;
Oracle Database also lets you rename column constraints.
ALTER TABLE dept
    RENAME CONSTRAINT dname_ukey TO dname_unikey;

ALTER TABLE hr.admin_emp DROP COLUMN sal;
ALTER TABLE hr.admin_emp DROP (bonus, commission);
If you enable OLTP compression on a table, you can drop table columns.
If you enable basic compression only, you cannot drop columns.

ALTER TABLE hr.admin_emp SET UNUSED (hiredate, mgr);
USER_UNUSED_COL_TABS,  ALL_UNUSED_COL_TABS ，dba_UNUSED_COL_TABS

ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;