11-Oracle学习_修改表结构-数据字典表

修改表结构

alter

drop

一, alter

1, 字段

① 添加一个字段

SQL> alter table stu add ( addr varchar2(100) );

② 删除一个字段

SQL> alter table stu drop (addr);

③ 修改现有字段

SQL> alter table stu modify ( addr varchar2(200) );

注: 如果已有数据, 则不能损失数据的精度及大小

2, 约束

① 删除一个约束

delete from class where id = 1

ORA-02292: 违反完整约束条件 (SCOTT.STU_CLASS_FK) - 已找到子记录

SQL> alter table stu drop constraint stu_class_fk;

SQL> delete from class where id = 1;

已删除 1 行。

② 添加一个约束

SQL> alter table stu add constraint stu_class_fk foreign key (class) references class (id);

二, drop

drop table table_name;

SQL> create table emp2 as select * from emp;

表已创建。

SQL> drop table emp2;

表已删除。

三, 数据字典表

0, dict 数据字典表 的 表

TABLE_NAME 数据字典表名

COMMENTS 描述

1, user_tables

当前用户下所有表的描述

SQL> desc user_tables;

SQL> select * from dict where table_name = 'USER_TABLES';

SQL> select table_name from user_tables;

2, user_views

Description of the user's own views

SQL> desc user_views;

SQL> select * from dict where table_name = 'USER_VIEWS';

SQL> select view_name from user_views;

3, user_constraints

Constraint definitions on user's own tables

SQL> desc user_constraints

SQL> select * from dict where table_name = 'USER_CONSTRAINTS';

cat tab

SQL> desc cat

Name Null? Type

----------- --------- -----------

TABLE_NAME NOT NULL VARCHAR2(30)

TABLE_TYPE VARCHAR2(11)

SQL> desc tab

Name Null? Type

----------- --------- -----------

TNAME NOT NULL VARCHAR2(30)

TABTYPE VARCHAR2(7)

CLUSTERID NUMBER

SQL> select * from dict where table_name='CAT';

TABLE_NAME COMMENTS

---------- -----------------------------------

CAT Synonym for USER_CATALOG

SQL> select * from dict where table_name='USER_CATALOG';

TABLE_NAME COMMENTS

-------------------- -------------------------

USER_CATALOG Tables, Views, Synonyms and Sequences owned by the user