1. Introduction to primary key constraints
- Primary key: One or more fields in the data table, used to uniquely identify a record in the data table.
2. Add primary key constraints
2.1 Add constraints when creating a table
Writing 1:
CREATE TABLE tb_students(
stu_num char(5) primary key,
stu_name varchar(10) not null,
stu_sex char(1) not null,
stu_age number(2) not null,
stu_tel char(11) not null
);
Writing method 2 (written in the back):
CREATE TABLE tb_students(
stu_num char(5),
stu_name varchar(10) not null,
stu_sex char(1) not null,
stu_age number(2) not null,
stu_tel char(11) not null,
primary key(stu_num)
);
2.2 Add constraints after creating the table
Change the following to: ADD CONSTRAINTS
For example: You have now created a student information table td_students without constraints
CREATE TABLE tb_students(
stu_num char(5),
stu_name varchar(10) not null,
stu_sex char(1) not null,
stu_age number(2) not null,
stu_tel char(11) not null
);
Now we can add the primary key to stu_nums in the td_students table with the following code:
ALTER TABLE tb_students ADD CONSTRAINTS pk_students PRIMARY KEY(stu_num);
3. Combined primary key
- Combined primary key: use 2 or more fields as the primary key
4. Add a composite primary key
4.1 Add a combined primary key when creating a table
Create a grade table tb_grades
CREATE TABLE tb_grades(
course_id char(3),
course_name varchar2(50),
stu_num char(5),
stu_name varchar2(10),
score number(3),
primary key(course_id,stu_num)
);
4.2 Create the table first, then add the joint primary key
Change the following to: ADD CONSTRAINTS
First create a score table without constraints:
CREATE TABLE tb_grades(
course_id char(3),
course_name varchar2(50),
stu_num char(5),
stu_name varchar2(10),
score number(3)
);
add constraints
ALTER TABLE tb_grades ADD CONSTRAINTS pk_grades PRIMARY KEY(course_id,stu_num);