What is a primary key in databases?

A primary key is a column or a set of columns in a database table that uniquely identifies each row in that table.

Main characteristics:

• Uniqueness: Each value of the primary key must be unique within the table.
• Not null (NOT NULL): The primary key cannot contain a NULL value.
• Stability: It is desirable that the primary key values do not change.

Purpose:

• Ensuring data integrity (entity integrity).
• Establishing relationships between tables (foreign keys reference primary keys).
• Query optimization (often used for indexing).

Example of creating a table with a primary key in SQL:

CREATE TABLE Customers (
    CustomerID int NOT NULL, -- Column for primary key
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    CONSTRAINT PK_Customer PRIMARY KEY (CustomerID) -- Defining primary key
);

A primary key can be composite, consisting of multiple columns:

CREATE TABLE OrderItems (
    OrderID int NOT NULL,
    ProductID int NOT NULL,
    Quantity int,
    CONSTRAINT PK_OrderItem PRIMARY KEY (OrderID, ProductID) -- Composite primary key
);

Auto-incrementing values for primary keys are often used (for example, auto-increment).

CREATE TABLE Products (
    ProductID int NOT NULL IDENTITY(1,1), -- Auto-increment primary key
    ProductName varchar(255) NOT NULL,
    CONSTRAINT PK_Product PRIMARY KEY (ProductID)
);