PRIMARY KEY – Unique + Not NULL

PRIMARY KEY – Unique + Not NULL
Because every row in your table deserves to be one of a kind.

🔧 Quick Syntax to Create a PRIMARY KEY

CREATE TABLE table_name (
   column_name datatype PRIMARY KEY
);

-- For multiple columns (composite key)
CREATE TABLE table_name (
   column1 datatype,
   column2 datatype,
   PRIMARY KEY (column1, column2)
);

This tells SQL: “Make sure this column (or combo) is never empty and never duplicated.”

Real-Life Analogy: A Classroom Without Student IDs

Imagine you track student grades like this:

Without a PRIMARY KEY

name	grade
John	A
Sarah	B
John	C

Two Johns — no idea who’s who. It’s like a class without roll numbers.

Let’s Fix It: Add a PRIMARY KEY

CREATE TABLE students (
   student_id INT PRIMARY KEY,
   name VARCHAR(50),
   grade CHAR(1)
);

INSERT INTO students (student_id, name, grade) VALUES
(1, 'John', 'A'),
(2, 'Sarah', 'B'),
(3, 'John', 'C');

Output:

student_id	name	grade
1	John	A
2	Sarah	B
3	John	C

Already Have a Table? Add PRIMARY KEY

ALTER TABLE students
ADD PRIMARY KEY (student_id);

Make sure the column is already NOT NULL and Unique.

Composite PRIMARY KEY – One Key, Two Columns

Use when no single column is unique, but a combo is.

Example Table:

student_id	subject_id	grade
1	101	A
1	102	B
2	101	C

CREATE TABLE results (
   student_id INT,
   subject_id INT,
   grade CHAR(1),
   PRIMARY KEY (student_id, subject_id)
);

Each row is unique by the combo of student + subject.

Don’t Do This:

-- Invalid! You can't have two primary key statements
PRIMARY KEY (student_id);
PRIMARY KEY (subject_id);

Only one PRIMARY KEY is allowed per table — it can include one or multiple columns, but must be defined in one line.

Summary Table – What to Know

Rule / Feature	What It Means
Unique & Not Null	SQL enforces both automatically
Only One PRIMARY KEY	One per table (can include multiple columns)
Composite Keys Allowed	Yes — when one column isn’t enough
Adds an Index	Speeds up lookups and joins
Enables FOREIGN KEYs	Links to other tables
Keeps Data Safe	Prevents duplicates, ensures structure

Recap – You Were Here

You started with a table full of duplicates
No reliable way to update or find rows
Now you know how to create, add, and use PRIMARY KEYs

Next up? Want to learn about FOREIGN KEYS, AUTO_INCREMENT, or compare PRIMARY KEY vs UNIQUE? Let’s keep building the SQL series!