Oracle外键(Foreign Key)用法详解（一）

1.目标

演示如何在Oracle数据库中使用外键

2.什么是外键？

1）在Oracle数据库中，外键是用来实现参照完整性的方法之一。打个形象的比喻，外键是指定义外键的表的列的值必须在另一个表中出现。

2）被参照的表称之为父表（parent table)，创建外键的表称之为子表（child table)。子表中的外键关联了父表中的主键。

3）外键可以在创建表时定义或者通过ALTER TABLE语句创建。

3.创建表时定义外键

语法：

CREATE TABLE table_name
(
   column1 datatype null/not null,
   column2 datatype null/not null,
   ...

   CONSTRAINT fk_column FOREIGN KEY  (column1,column2,... column_n) REFERENCES parent_table (column1,column2,...column_n)
);

示例1：基于单列的外键

create table tb_supplier
(
  supplier_id number not null,
  supplier_name varchar2(50) not null,
  contact_name varchar2(50),
  CONSTRAINT pk_supplier PRIMARY KEY (supplier_id)
);

create table tb_products
(
  product_id number not null,
  product_name varchar2(100),
  supplier_id number not null,
  constraint fk_products_supplier foreign key (supplier_id) references tb_supplier(supplier_id)
);

示例2：基于多列的外键

drop table TB_PRODUCTS;
drop table TB_SUPPLIER;

create table tb_supplier
(
  supplier_id number not null,
  supplier_name varchar2(50) not null,
  contact_name varchar2(50),
  CONSTRAINT pk_supplier PRIMARY KEY (supplier_id,supplier_name)
);

create table tb_products
(
  product_id number not null,
  product_name varchar2(100),
  supplier_name varchar2(50),
  supplier_id number not null,
  constraint fk_products_supplier foreign key (supplier_id,supplier_name) references tb_supplier(supplier_id,supplier_name)
);

4.使用ALTER TABLE命令创建外键

语法：

ALTER TABLE table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY (column1, column2,...column_n)
REFERENCES parent_table (column1,column2,...column_n);

示例：

drop table TB_PRODUCTS;
drop table TB_SUPPLIER;

create table tb_supplier
(
  supplier_id number not null,
  supplier_name varchar2(50) not null,
  contact_name varchar2(50),
  CONSTRAINT pk_supplier PRIMARY KEY (supplier_id,supplier_name)
);

create table tb_products
(
  product_id number not null,
  product_name varchar2(100),
  supplier_name varchar2(50),
  supplier_id number not null
);

--使用alter table创建外键
 alter table tb_products
 add constraint fk_products_supplier
 foreign key (supplier_id,supplier_name)
 references tb_supplier(supplier_id,supplier_name);

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