create table test(a varchar2(20),b number(2))
重名命
alter table test rename to test1
（rename test1 to test2）
alter table test2 add (d varchar2(30),e number(4))
alter table test2 rename column e to f
重名命alter table test rename to test1（rename test1 to test2）
增删字段
alter table test2 drop column f
修改字段类型和大小alter table test2 modify d varchar(40)alter table test2 modify f varchar(40)
alter table test2 add c varchar2(30)
alter table test2 add (d varchar2(30),e number(4))alter table test2 rename column e to f
处理约束
alter table test2 add constraint pk_mypri primary key (d,f)
利用下面的语句在删除DEPT表中的PRIMARY KEY约束时，同时将删除其它表中引用这个约束的FOREIGN KEY约束:
alter table dept drop primary key cascade
alter table dept drop primary key cascade
加foreign key约束(多字段／表级）
alter table employees add constraint emp_jobs_fk foreign key (job,deptno) references jobs (jobid,deptno) on delete cascade
alter table employees add constraint emp_jobs_fk foreign key (job,deptno) references jobs (jobid,deptno) on delete cascade