oracle alter table详解

//建测试表
create table dept(
       deptno number(3) primary key,
       dname varchar2(10),
       loc varchar2(13) 
       );
create table employee_info(
       empno number(3),
       deptno number(3),
       ename varchar2(10),
       sex char(1),
       phone number(11),
       address varchar2(50),
       introduce varchar2(100)
       );
--
//0.重命名
  //0.1 表:rename dept to dt;
           rename dt to dept;
  //0.2 列:alter table dept rename column loc to location;
           alter table dept rename column location to loc;
//1.添加约束
  //1.1 primary key
      alter table employee_info add constraint pk_emp_info primary key(empno);
  //1.2 foreign key
      alter table employee_info add constraint fk_emp_info foreign key(deptno)
      references dept(deptno);
  //1.3 check
      alter table employee_info add constraint ck_emp_info check
      (sex in ('F','M'));
  //1.4 not null
      alter table employee_info modify phone constraint not_null_emp_info not null;
  //1.5 unique
      alter table employee_info add constraint uq_emp_info unique(phone);
  //1.6 default
      alter table employee_info modify sex char(2) default 'M';
//2.添加列
   alter table employee_info add id varchar2(18);
   alter table employee_info add hiredate date default sysdate not null;
//3.删除列
   alter table employee_info drop column introduce;
//3.修改列
  //3.1 修改列的长度
      alter table dept modify loc varchar2(50);
  //3.2 修改列的精度
      alter table employee_info modify empno number(2);
  //3.3 修改列的数据类型
      alter table employee_info modify sex char(2);
  //3.4 修改默认值
      alter table employee_info modify hiredate default sysdate+1;
//4.禁用约束
  alter table employee_info disable constraint uq_emp_info;
//5.启用约束
  alter table employee_info enable constraint uq_emp_info;
//6.延迟约束
  alter table employee_info drop constraint fk_emp_info;
  alter table employee_info add constraint fk_emp_info foreign key(deptno)
        references dept(deptno)
  deferrable initially deferred;
//7.向表中添加注释
  comment on table employee_info is 'information of employees';
//8.向列添加注释
  comment on column employee_info.ename is 'the name of employees';
  comment on column dept.dname is 'the name of department';
//9.清除表中所有数据
  truncate table employee_info;
//10.删除表
  drop table employee_info;
--
//下面来看看刚刚才我们对表dept和表employee_info所做的更改
//user_constraints视图里面包含了刚刚才我们创建的所有约束，以及其他信息，
//你可以用desc user_constraints命令查看其详细说明
select constraint_name,constraint_type,status,deferrable,deferred
from user_constraints
where table_name='EMPLOYEE_INFO';
--
CONSTRAINT_NAME                CONSTRAINT_TYPE STATUS   DEFERRABLE     DEFERRED
------------------------------ --------------- -------- -------------- ---------
PK_EMP_INFO                    P               ENABLED  NOT DEFERRABLE IMMEDIATE
FK_EMP_INFO                    R               ENABLED  DEFERRABLE     DEFERRED
NOT_NULL_EMP_INFO              C               ENABLED  NOT DEFERRABLE IMMEDIATE
SYS_C005373                    C               ENABLED  NOT DEFERRABLE IMMEDIATE
UQ_EMP_INFO                    U               ENABLED  NOT DEFERRABLE IMMEDIATE
CK_EMP_INFO                    C               ENABLED  NOT DEFERRABLE IMMEDIATE
//我们可以通过user_cons_columns视图查看有关列的约束信息；
select owner,constraint_name,table_name,column_name
from user_cons_columns
where table_name='EMPLOYEE_INFO';
--
OWNER                          CONSTRAINT_NAME                TABLE_NAME                     COLUMN_NAME
------------------------------ ------------------------------ ------------------------------ ---------------
YEEXUN                         PK_EMP_INFO                    EMPLOYEE_INFO                  EMPNO
YEEXUN                         CK_EMP_INFO                    EMPLOYEE_INFO                  SEX
YEEXUN                         NOT_NULL_EMP_INFO              EMPLOYEE_INFO                  PHONE
YEEXUN                         SYS_C005373                    EMPLOYEE_INFO                  HIREDATE
YEEXUN                         UQ_EMP_INFO                    EMPLOYEE_INFO                  PHONE
YEEXUN                         FK_EMP_INFO                    EMPLOYEE_INFO                  DEPTNO
//我们将user_constraints视图与user_cons_columns视图连接起来
//查看约束都指向哪些列
column column_name format a15;
select ucc.column_name,ucc.constraint_name,uc.constraint_type,uc.status
from user_constraints uc,user_cons_columns ucc
where uc.table_name=ucc.table_name and
      uc.constraint_name=ucc.constraint_name and
      ucc.table_name='EMPLOYEE_INFO';
--
COLUMN_NAME     CONSTRAINT_NAME                CONSTRAINT_TYPE STATUS
--------------- ------------------------------ --------------- --------
EMPNO           PK_EMP_INFO                    P               ENABLED
DEPTNO          FK_EMP_INFO                    R               ENABLED
PHONE           NOT_NULL_EMP_INFO              C               ENABLED
HIREDATE        SYS_C005373                    C               ENABLED
PHONE           UQ_EMP_INFO                    U               ENABLED
SEX             CK_EMP_INFO                    C               ENABLED
--
//这里有个constraint_type，他具体指下面几种类型:
//C:check,not null
//P:primary key
//R:foreign key
//U:unique
//V:check option
//O:read only
--
//我们可以通过user_tab_comments视图获得对表的注释
select * from user_tab_comments
where table_name='EMPLOYEE_INFO';
TABLE_NAME                     TABLE_TYPE  COMMENTS
------------------------------ ----------- --------------------------
EMPLOYEE_INFO                  TABLE       information of employees
--
//我们还可以通过user_col_comments视图获得对表列的注释：
select * from  user_col_comments
where table_name='EMPLOYEE_INFO';
--
TABLE_NAME                     COLUMN_NAME                    COMMENTS
------------------------------ ------------------------------ ---------------------------
EMPLOYEE_INFO                  EMPNO                          
EMPLOYEE_INFO                  DEPTNO                         
EMPLOYEE_INFO                  ENAME                          the name of employees
EMPLOYEE_INFO                  SEX                            
EMPLOYEE_INFO                  PHONE                          
EMPLOYEE_INFO                  ADDRESS                        
EMPLOYEE_INFO                  ID                             
EMPLOYEE_INFO                  HIREDATE 
--
select * from user_col_comments
where table_name='EMPLOYEE_INFO' and
      comments is not null;
--
TABLE_NAME                     COLUMN_NAME                    COMMENTS
------------------------------ ------------------------------ ------------------------
EMPLOYEE_INFO                  ENAME                          the name of employees
--
//最后我们来查看一下修改后的表:
desc employee_info;
Name     Type         Nullable Default   Comments              
-------- ------------ -------- --------- --------------------- 
EMPNO    NUMBER(2)                                             
DEPTNO   NUMBER(3)    Y                                        
ENAME    VARCHAR2(10) Y                  the name of employees 
SEX      CHAR(2)      Y        'M'                             
PHONE    NUMBER(11)                                            
ADDRESS  VARCHAR2(50) Y                                        
ID       VARCHAR2(18) Y                                        
HIREDATE DATE                  sysdate+1
--
desc dept;
Name   Type         Nullable Default Comments               
------ ------------ -------- ------- ---------------------- 
DEPTNO NUMBER(3)                                            
DNAME  VARCHAR2(10) Y                the name of department 
LOC    VARCHAR2(50) Y
--