Complete guide with definitions, differences, SQL examples, advanced scenarios, and FAQs — in English + Hindi (Hybrid) — 2025 Updated.
Relational databases depend on structured relationships to keep data accurate and meaningful. Two pillars of that structure are the Primary Key (PK) and the Foreign Key (FK). PK ensures each row is uniquely identifiable; FK connects tables and enforces referential integrity. This hybrid guide (English + Hindi) helps you master both concepts and apply them in real projects.
A Primary Key is a column or set of columns that uniquely identifies each row in a table. It must be unique and not null. Most DB engines automatically index primary keys for fast lookups.
CREATE TABLE Students (
student_id INT PRIMARY KEY,
name VARCHAR(100),
dob DATE
);
A Foreign Key is a column (or set) in one table that refers to the Primary Key (or a UNIQUE key) in another table. It maintains relationships and enforces referential integrity.
CREATE TABLE Orders (
order_id INT PRIMARY KEY,
customer_id INT,
order_date DATE,
FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);
| Feature | Primary Key | Foreign Key |
|---|---|---|
| Uniqueness | Unique & NOT NULL | May be duplicate / NULL |
| Purpose | Identify rows | Link tables |
| Per table | One (composite allowed) | Multiple allowed |
| Indexing | Usually indexed | May not be indexed |
| Integrity | No | Enforces referential integrity |
-- Customers table
CREATE TABLE Customers (
customer_id INT PRIMARY KEY,
name VARCHAR(100),
email VARCHAR(100) UNIQUE
);
-- Orders table referencing Customers
CREATE TABLE Orders (
order_id INT PRIMARY KEY,
customer_id INT,
order_date DATE,
amount DECIMAL(10,2),
FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);
-- OrderItems with cascade delete
CREATE TABLE OrderItems (
item_id INT PRIMARY KEY,
order_id INT,
product_id INT,
qty INT,
FOREIGN KEY (order_id) REFERENCES Orders(order_id) ON DELETE CASCADE
);
-- Example: child table where PK is also FK
CREATE TABLE EmployeeDetails (
employee_id INT PRIMARY KEY,
details JSON,
FOREIGN KEY (employee_id) REFERENCES Employees(employee_id)
);
Customers → Orders → OrderItems. Use FK to link orders to customers; cascade rules for order cleanup.
Accounts → Transactions. PK for account numbers; FK in transactions to maintain ledger integrity.
Students → Enrollments → Courses. Composite PK for enrollment (student_id, course_id).
Q: Primary Key और Foreign Key में क्या अंतर है?
A: Primary Key तालिका में हर रिकॉर्ड को यूनिक पहचान देता है; Foreign Key तालिकाओं को जोड़ता है और referential integrity लागू करता है.
Q: क्या Foreign Key NULL हो सकती है?
A: हाँ, जब तक NOT NULL constraint न लगाया गया हो.
Q: क्या एक तालिका में एक से अधिक Foreign Keys हो सकती हैं?
A: हाँ — एक तालिका में कई FK हो सकते हैं जो विभिन्न parent तालिकाओं को संदर्भित करते हैं.
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Yes. In a different table, the Primary Key value can appear as a Foreign Key. In the same table, a column can participate in both PK and FK roles (e.g., self-referencing hierarchies) but typically as part of different constraints.
CREATE TABLE Employees (
emp_id INT PRIMARY KEY,
name VARCHAR(100),
manager_id INT,
CONSTRAINT fk_emp_manager
FOREIGN KEY (manager_id) REFERENCES Employees(emp_id)
);
Here, emp_id is the PK. The manager_id column is an FK referencing the same table’s PK.
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),
CONSTRAINT fk_profile_user
FOREIGN KEY (user_id) REFERENCES Users(user_id)
);
UserProfiles.user_id is both PK and FK, enforcing a strict 1-to-1 relation.
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
);
Note: ON DELETE SET NULL allows FK to be NULL when parent is deleted.
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');
SELECT o.order_id, c.name, o.order_date
FROM Orders o
JOIN Customers c
ON o.customer_id = c.customer_id;
Targets: primary key and foreign key in SQL with examples, sql primary key foreign key, how to use primary key and foreign key in SQL
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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:
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