Primary Key in SQL

A Primary Key is a column or a set of columns in a table that uniquely identifies each row in that table. It ensures uniqueness and non-nullability for the key columns.

Key Features of Primary Key:

• Uniquely identifies each record.

• Cannot contain NULL values.

• Only one primary key per table.

• Often used as a reference by foreign keys in other tables.

How to Define a Primary Key

1. While Creating a Table:

CREATE TABLE employees (    employee_id INT PRIMARY KEY,    name VARCHAR(100),    department VARCHAR(50));

2. Using Composite Primary Key (multiple columns):

CREATE TABLE orders (    order_id INT,    product_id INT,    quantity INT,    PRIMARY KEY (order_id, product_id));

3. Add Primary Key to Existing Table:

ALTER TABLE employeesADD PRIMARY KEY (employee_id);

Example

CREATE TABLE users (    user_id INT PRIMARY KEY,    username VARCHAR(50) NOT NULL,    email VARCHAR(100));

Notes:

• Primary key automatically creates a unique index.

• You cannot insert duplicate or NULL values in the primary key column(s).

• If you try to add a primary key on columns with duplicates or NULLs, it will fail.

Want help with foreign keys, or auto-increment primary keys?