SQL Primary Key

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A Primary Key is a column (or a group of columns) in a table that uniquely identifies each row (record) in that table.
It ensures that no two rows have the same value in that column.

Key Characteristics of a Primary Key
1. Unique
   • Every value must be different.
2. Not NULL
   • It cannot contain empty or NULL values.
3. One Primary Key per Table
   • A table can have only one primary key, but it can contain multiple columns (called a composite key).
4. Used to Create Relationships
   • Other tables can reference it using Foreign Keys.

Creating a Primary Key in SQL Example 1: While Creating Table

CREATE TABLE Students (
    Student_ID INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT
);

Example 2: Primary Key with AUTO INCREMENT

CREATE TABLE Students (
    Student_ID INT IDENTITY(1,1) PRIMARY KEY,
    Name VARCHAR(50),
    Age INT
);

Example 3: Composite Primary Key

CREATE TABLE StudentCourses (
    Student_ID INT,
    Course_ID INT,
    PRIMARY KEY (Student_ID, Course_ID)
);

Sometimes two columns together create uniqueness.
Here the combination of both columns is the Primary Key.

Rules for Primary Key
A primary key:
   • Cannot contain duplicate values
   • Cannot contain NULL values
   • Should be stable (not change often)

In simple words:
A Primary Key is the unique ID of each record in a table.

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