Unique Constraints for Oracle Database

The role of a unique constraint

Fields are guaranteed to be unique.

 

The difference between a unique constraint and a primary key constraint

Primary key field value must be non-null.

Unique constraints allow a null value.

There can only be one primary key in each table.

There can be more than one unique constraint per table.

 

3. Set a unique constraint when creating a table

1. Create a unique constraint at the column level

grammar:

CREATE TABLE table_name

(column_name datatype UNIQUE,...)

Code:

1. SQL> create table userinfo_u
2. 2(id varchar2(10) primary key,
3. 3 username varchar2(20) unique,
4. 4 userpwd varchar2(20));
6. 表已创建。

2. Create a unique constraint at the table level

CONSTRAINT constraint_name

UNIQUE(column_name)

1. SQL> create table userinfo_u1
2. 2(id varchar2(10) primary key,
3. 3 username varchar2(20),
4. 4 constraint un_username unique(username));
6. 表已创建。