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SQL PRIMARY KEY Constraint
SQL Learning
SQL PRIMARY KEY Constraint
Learn how PRIMARY KEY ensures each row in your table is uniquely identifiable. Essential for relational database design.
What Is a PRIMARY KEY in SQL?
A PRIMARY KEY is a column—or a combination of columns—that uniquely identifies each row in a table. It prevents duplicate values and ensures every row can be referenced reliably.
Key Properties
- Must be unique across all rows
- Cannot contain
NULLvalues - Each table can have only one
PRIMARY KEY
Defining a PRIMARY KEY
Inline Definition
CREATE TABLE Users (
user_id INTEGER PRIMARY KEY,
username TEXT NOT NULL
);Table-Level Definition
CREATE TABLE Videos (
video_id INTEGER,
title TEXT,
upload_date DATE,
PRIMARY KEY (video_id)
);Composite Primary Key
Use more than one column to uniquely identify rows.
CREATE TABLE Interactions (
user_id INTEGER,
video_id INTEGER,
interaction_type TEXT,
PRIMARY KEY (user_id, video_id)
);Why PRIMARY KEYs Matter
- They enable fast lookups and indexing
- They enforce data integrity—no duplicates allowed
- They support relationships via
FOREIGN KEYs
Tips for Using PRIMARY KEYs
- Use an
INTEGERprimary key for simplicity and performance - Avoid using
emailorusernameas a primary key—they can change! - Use a surrogate key (like
id) and keep natural keys as unique constraints - Index your foreign key columns for faster joins