What is RDBMS

Last updated: October 19, 2026Oct 19, 2026 6 min read

An RDBMS (Relational Database Management System) is software that stores data in related tables and lets you work with it using SQL. MySQL is one of the world's most popular RDBMS products, alongside PostgreSQL, Oracle Database, Microsoft SQL Server and MariaDB.

In this tutorial you will learn the building blocks of an RDBMS: tables, keys, relationships, integrity rules, normalization and ACID transactions, each with a small MySQL example based on a library.

1. What is an RDBMS?

The relational model was described by E. F. Codd at IBM in 1970. Its key idea: store everything in relations (tables), and link tables through matching values rather than physical pointers. An RDBMS is a DBMS that implements this model.

2. DBMS vs RDBMS

3. Keys: How Rows Are Identified and Linked

• Primary key (PK): a column (or set of columns) that uniquely identifies each row and is never NULL, such as member_id.
• Candidate key: any column that could be the primary key, such as a unique email.
• Foreign key (FK): a column that refers to the primary key of another table, such as loans.member_id.
• Composite key: a key made of two or more columns together.
• Surrogate vs natural key: an invented number (AUTO_INCREMENT) versus a real-world value (ISBN).

4. Building the Library Database

Here are the three tables from Figure 1 in MySQL, with their keys:

CREATE DATABASE city_library;
USE city_library;

CREATE TABLE members (
  member_id INT PRIMARY KEY,
  name      VARCHAR(40) NOT NULL,
  email     VARCHAR(80) UNIQUE
);

CREATE TABLE books (
  book_id INT PRIMARY KEY,
  title   VARCHAR(80) NOT NULL,
  copies  INT CHECK (copies >= 0)
);

CREATE TABLE loans (
  loan_id   INT PRIMARY KEY,
  member_id INT NOT NULL,
  book_id   INT NOT NULL,
  due_date  DATE,
  FOREIGN KEY (member_id) REFERENCES members(member_id),
  FOREIGN KEY (book_id)   REFERENCES books(book_id)
);

INSERT INTO members VALUES (1,'Kavya','kavya@example.com'),(2,'Aman','aman@example.com'),(3,'Sara',NULL);
INSERT INTO books   VALUES (12,'Dune',3),(15,'Emma',1),(18,'Ulysses',2);
INSERT INTO loans   VALUES (101,1,12,'2026-03-10'),(102,1,15,'2026-03-12'),(103,3,12,'2026-03-15');

Because tables are linked by keys, one JOIN query can answer "who borrowed what?":

SELECT m.name, b.title, l.due_date
FROM loans l
JOIN members m ON m.member_id = l.member_id
JOIN books   b ON b.book_id   = l.book_id
ORDER BY l.due_date;

Result: 3 rows

5. Types of Relationships

6. Integrity Rules the RDBMS Enforces

An RDBMS refuses data that breaks the rules. This is called data integrity.

Entity integrity: the primary key must be unique and not NULL.

INSERT INTO members VALUES (1, 'Rahul', 'rahul@example.com');

ERROR 1062 (23000): Duplicate entry '1' for key 'members.PRIMARY'

Referential integrity: a foreign key must point to a row that exists. There is no member 9:

INSERT INTO loans VALUES (104, 9, 18, '2026-03-20');

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`city_library`.`loans`, CONSTRAINT `loans_ibfk_1` FOREIGN KEY (`member_id`) REFERENCES `members` (`member_id`))

And you can't delete a member who still has loans, because that would leave loans pointing at nobody:

DELETE FROM members WHERE member_id = 1;

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`city_library`.`loans`, CONSTRAINT `loans_ibfk_1` FOREIGN KEY (`member_id`) REFERENCES `members` (`member_id`))

Domain integrity: values must fit the column's type and checks:

UPDATE books SET copies = -1 WHERE book_id = 18;

ERROR 3819 (HY000): Check constraint 'books_chk_1' is violated.

7. Normalization in Brief

Normalization is the process of organising tables so that each fact is stored once. Imagine one big table where every loan repeats the member's name and email. If Kavya changes her email, you must update many rows, and missing one leaves the data inconsistent.

8. ACID Transactions

An RDBMS groups changes into transactions with four guarantees: Atomicity, Consistency, Isolation and Durability. Lending a book needs two changes: add a loan and reduce the available copies. They must happen together:

START TRANSACTION;
INSERT INTO loans VALUES (104, 2, 18, '2026-03-22');
UPDATE books SET copies = copies - 1 WHERE book_id = 18;
COMMIT;

SELECT book_id, title, copies FROM books WHERE book_id = 18;

Result: 1 row

If anything fails before COMMIT, a ROLLBACK undoes both changes, so you never get a loan without the stock going down. See MySQL Transaction for details.

9. Advantages and Limitations

10. Practice Exercises

SELECT m.name
FROM members m
LEFT JOIN loans l ON l.member_id = m.member_id
WHERE l.loan_id IS NULL;

SELECT b.title, COUNT(l.loan_id) AS times_borrowed
FROM books b
LEFT JOIN loans l ON l.book_id = b.book_id
GROUP BY b.book_id, b.title
ORDER BY times_borrowed DESC, b.title;

11. Summary

• An RDBMS stores data in related tables and is queried with SQL. MySQL is a popular example.
• Primary keys identify rows; foreign keys link tables.
• Relationships can be one-to-one, one-to-many or many-to-many (through a bridge table).
• The RDBMS enforces entity, referential and domain integrity.
• Normalization stores each fact once; joins combine the tables again.
• ACID transactions keep related changes all-or-nothing.