Integrity constraints

1. Primary key constraint: primary key
2. Auto-increment key constraint: auto_increment
3. Unique key constraint: unique
4. Not null constraint: not null
5. Default value constraint: default
6. Foreign key constraint: foreign key

Only one primary key can be created in a table, but there can be multiple unique keys.

Example of usage:

CREATE TABLE user(
id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
nickname varchar(50) UNIQUE NOT NULL,
age TINYINT UNSIGNED NOT NULL DEFAULT 18,
sex ENUM('male','female'));

Relational database table design

1. One to One

As shown in the figure, these are two tables, one for basic user information and one for identity information.

If you want to associate the two tables, you need to add a field to the identity information table

2. One-to-many

For example, to make an e-commerce system

• User User
• Product Product
• Order Order

Analysis:
Users and products: no relationship
Users and orders: one-to-many relationship
products and orders: many-to-many relationship

One-to-many relationship: Add a column to the order sub-table to associate the primary key of the parent table (the field representing the user id).But in this way, it is obvious that data redundancy, especially with hundreds or thousands of orders, will lead to large batches of modifications.

3. Many-to-many

In order to solve the problem of data redundancy, an intermediate table can be created.