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ORACLE五种约束的使用
1、唯一性约束
添加惟一性约束
ALTER TABLE USERS ADD CONSTRAINT UQ_EMAIL UNIQUE (EMAIL)
删除惟一性约束
ALTER TABLE USERS DROP CONSTRAINT UQ_EMAIL
重命名惟一性约束
ALTER TABLE USERS RENAME CONSTRAINT SYS_C005188 TO UQ_EMAIL
禁用/启用惟一性约束
ALTER TABLE USERS DISABLE CONSTRAINT UQ_EMAIL
2、非空约束
方式1.在创建表的时候就约束非空,如下:
create table student (
id number(5) primary key,
name varchar2(20) not null
);
方式2.对于已存在的表添加非空约束,如下:
alter table student modify name not null;
3、主键约束
为表添加多列主键
ALTER TABLE EMP ADD CONSTRAINT PK_EMP PRIMARY KEY(EMP_ID,EMP_NAME)
4、外键约束
CREATE TABLE CUSTOMERS
(
CUSTOMER_ID NUMBER,
CUSTOMER_NAME VARCHAR(20),
CONSTRAINT PK_CUSTOMERS PRIMARY KEY(CUSTOMER_ID)
)
CREATE TABLE ORDERS
(
ORDER_ID NUMBER,
CUSTOMER_ID NUMBER,
PRODUCT_NAME VARCHAR(20),
QUANTITY NUMBER,
CONSTRAINT PK_ORDERS PRIMARY KEY(ORDER_ID)
)
ALTER TABLE ORDERS ADD CONSTRAINT FK_ORDERS_CUSTOMERS FOREIGN KEY(ORDER_ID ) REFERENCES CUSTOMERS(CUSTOMER_ID)
5、检查约束
添加检查约束
ALTER TABLE STUDENTS ADD CONSTRAINT CK_STUDENT_NAME CHECK (LENGTH(STUDENT_NAME) <= 4)
ALTER TABLE STUDENTS ADD CONSTRAINT CK_SEX CHECK (SEX IN ('男','女'))