Скачать или смотреть Set Unique key, Primary key and Foreign key in Mysql Database | Key set in mysql | Part-04

To set unique keys, primary keys, and foreign keys in a MySQL database, you'll typically use the ALTER TABLE statement along with specific keywords to define these constraints. Below are examples for each type of key:

1. Unique Key:
To set a unique key on one or multiple columns, you can use the UNIQUE keyword:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column_name);
Example:

ALTER TABLE users
ADD CONSTRAINT unique_email UNIQUE (email);
This command adds a unique constraint named unique_email on the email column in the users table.

2. Primary Key:

To set a primary key on one or multiple columns, you use the PRIMARY KEY keyword:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name PRIMARY KEY (column_name);
Example:

ALTER TABLE users
ADD CONSTRAINT pk_user_id PRIMARY KEY (user_id);
This command adds a primary key constraint named pk_user_id on the user_id column in the users table.

3. Foreign Key:

To set a foreign key relationship between two tables, you use the FOREIGN KEY keyword along with REFERENCES to specify the referenced table and column:

ALTER TABLE child_table
ADD CONSTRAINT constraint_name FOREIGN KEY (column_name)
REFERENCES parent_table(parent_column_name);
Example:

ALTER TABLE orders
ADD CONSTRAINT fk_customer_id FOREIGN KEY (customer_id)
REFERENCES customers(customer_id);
This command adds a foreign key constraint named fk_customer_id on the customer_id column in the orders table, referencing the customer_id column in the customers table.

Make sure to replace table_name, constraint_name, column_name, parent_table, parent_column_name, etc., with your actual table and column names.

Additionally, it's important to ensure that the data types of the columns used in key constraints match between the referencing and referenced tables for foreign keys.