05|Oracle learning (UNIQUE constraint)

1. Introduction to UNIQUE constraints

• Also called: unique key constraint, used to limit the uniqueness of field values ​​in the data table.
  

1.1 Difference between UNIQUE and primary key:

• There is only one primary key/combined primary key per table.
• UNIQUE constraints can exist in multiple fields in a table. For example: the student's phone number and ID number are unique.

2. Add a unique constraint

2.1 Add when creating a table

2.1.1 Case

• Create a student information table and set the phone number as the only constraint:

create table tb_students(
    stu_num char(5) not null,
    stu_name varchar(10) not null,
    stu_sex char(1) not null,
    stu_age number(2) not null,
    stu_tel char(11) not null,
    constraint uq_student_tel UNIQUE(stu_tel)
);

In actual development, the following are commonly used, just add a unique directly after stu_tel:

create table tb_students(
    stu_num char(5) not null,
    stu_name varchar(10) not null,
    stu_sex char(1) not null,
    stu_age number(2) not null,
    stu_tel char(11) not null unique
);

2.2 After creating the table, add it

2.2.1 Case

• Create a student information table without a unique key:

create table tb_students(
    stu_num char(5) not null,
    stu_name varchar(10) not null,
    stu_sex char(1) not null,
    stu_age number(2) not null,
    stu_tel char(11) not null
);

• Then add a unique key to the table: stu_tel:

alter table tb_students
add constraint uq_student_tel
unique(stu_tel);

3. Remove the unique constraint

3.1 Case

• Drop the unique constraint uq_student_tel:

alter table tb_students
drop constraint uq_student_tel;