Clear definitions, side-by-side comparison, practical SQL examples, and answers to common questions like can a primary key be a foreign key or can a foreign key be NULL. Hybrid English + Hindi guide — 2025 update.
Primary Keys (PK) uniquely identify rows; Foreign Keys (FK) link tables and enforce referential integrity. This section answers high-volume queries such as primary key and foreign key, difference between primary key and foreign key, and primary key vs foreign key in SQL.
A Primary Key (PK) uniquely identifies each row in a table. It must be unique and NOT NULL. Databases typically index PKs for both integrity and performance.
A Foreign Key (FK) is a column in one table that references the Primary Key (or a UNIQUE key) of another table to enforce relationships and referential integrity.
Side-by-side comparison for quick answers to search queries.
| Aspect | Primary Key | Foreign Key |
|---|---|---|
| Uniqueness | Unique & NOT NULL | Nullable & may repeat |
| Purpose | Identify rows | Link tables |
| Count per table | One (composite allowed) | Multiple allowed |
| Enforces | Entity integrity | Referential integrity |
Yes — in some designs (1:1 relationships or identifying/weak-entity models) a primary key can also serve as a foreign key.
NULL: Yes, unless NOT NULL is specified.
Duplicate: Yes — multiple child rows can reference the same parent row (answers queries like “is foreign key unique?”).
Primary Key = unique identifier (NOT NULL). Foreign Key = reference to parent (nullable/duplicates allowed). Design PK/FK carefully for integrity and joins.
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Primary Key vs Foreign Key — Quick Comparison ( primary key vs foreign key)
This comparison answers common queries like difference between primary key and foreign key,
is foreign key unique, and primary key vs foreign key in SQL. Use this as a quick reference
when designing database tables and ER diagrams.
This table targets queries like primary key vs foreign key, difference between primary and foreign key,
primary key foreign key example, and is foreign key unique. For practical SQL examples and ER-diagram tips, refer to the next section of the guide.
Many learners ask: can a primary key be a foreign key? The answer is Yes. In SQL database design, a primary key (PK) can also serve as a foreign key (FK), especially in 1-to-1 relationships or self-referencing hierarchies. Below are two practical examples with SQL code.
CREATE TABLE Employees ( emp_id INT PRIMARY KEY, name VARCHAR(100), manager_id INT, FOREIGN KEY (manager_id) REFERENCES Employees(emp_id) );
Here emp_id is the table’s Primary Key.
The manager_id column is a Foreign Key pointing back to the same PK, creating a hierarchical relationship.
CREATE TABLE Users ( user_id INT PRIMARY KEY, email VARCHAR(100) UNIQUE ); CREATE TABLE UserProfiles ( user_id INT PRIMARY KEY, -- also FK bio VARCHAR(200), FOREIGN KEY (user_id) REFERENCES Users(user_id) );
In this case UserProfiles.user_id is both Primary Key and Foreign Key, ensuring a strict one-to-one mapping with Users.
Keywords covered: primary key as foreign key, can primary key be foreign key, primary key foreign key example in SQL.
See SQL Examples
CREATE TABLE Customers (
customer_id INT PRIMARY KEY,
name VARCHAR(100) NOT NULL
);
CREATE TABLE Orders (
order_id INT PRIMARY KEY,
customer_id INT,
order_date DATE NOT NULL,
CONSTRAINT fk_orders_customer
FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
ON UPDATE CASCADE
ON DELETE SET NULL
);
Here, Orders.customer_id is a Foreign Key referencing Customers.customer_id.
ON DELETE SET NULL means the FK becomes NULL when the parent row is removed.
INSERT INTO Customers(customer_id, name)
VALUES (1, 'Asha'), (2, 'Ravi');
INSERT INTO Orders(order_id, customer_id, order_date)
VALUES (101, 1, '2025-08-01'),
(102, 2, '2025-08-05');
Always insert parent rows (Customers) before child rows (Orders) to satisfy FK constraints.
SELECT o.order_id, c.name, o.order_date
FROM Orders o
JOIN Customers c
ON o.customer_id = c.customer_id;
This SQL join retrieves order details along with customer names by using the Primary–Foreign Key relationship.
Keywords covered: primary key and foreign key in SQL with examples, sql primary key foreign key example, how to use primary key and foreign key in SQL, difference between primary and foreign key in SQL.
See how primary keys and foreign keys are used in production systems — from retail to healthcare. These practical examples clarify why primary key vs foreign key design matters for data integrity, reporting, and performance.
Use Customers.customer_id (PK) and Orders.customer_id (FK) to link purchases to buyers. FK cascade rules help keep order history consistent when customer records change.
Accounts use a PK (account_id) while Transactions reference it via an FK. Referential integrity ensures transactions cannot reference deleted accounts.
Model many-to-many relationships (Students ↔ Courses) with a join table using a composite PK (student_id, course_id) while each column references the parent PKs as FKs.
Patient records use a stable primary key (e.g., patient_id). Appointments, prescriptions and lab results reference this PK with FKs to maintain an auditable history.
In star schemas, dimension tables have PKs used as FKs in fact tables (e.g., dim_date.date_id → fact_sales.date_id). This makes aggregations and joins fast and consistent.
These use cases show why careful primary key and foreign key design is essential for data integrity, query performance, and maintainability. Apply appropriate cascade rules, indexing, and constraints based on business rules.
🎯 Learn Practical DB Design at Vista AcademyPrimary Keys (PK) and Foreign Keys (FK) are the foundation of relational databases. They ensure data integrity, prevent duplicates, and enable relationships between entities. Here are some common real-world scenarios:
Customers (PK) linked to Orders (FK). Ensures each order belongs to a valid customer and enables order history tracking.
Accounts (PK) connected to Transactions (FK). Guarantees that every transaction is tied to an existing account.
Students (PK) linked to Enrollments and Courses (FK). Maintains consistency in academic records and course registrations.
Shipments (PK) linked to Delivery Tracking (FK). Keeps data consistent across warehouses, routes, and delivery status updates.
Keywords: why do relational databases use primary keys and foreign keys, primary key foreign key examples, real-world use of primary and foreign key
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Now that you’ve mastered Primary & Foreign Keys, it’s time to practice advanced SQL concepts. Explore the following tutorials and apply them on real datasets to level up your skills:
🚀 Keep practicing on real-world datasets and refining your skills. The more hands-on experience you gain, the stronger your SQL foundation will become!