Natural Join in DBMS: Definition, Example, and Usage

A natural join is a type of join in databases that automatically combines rows from two tables based on all columns with the same names and compatible data types. It returns only one copy of each matching column, merging rows where these columns have equal values.

How It Works

Imagine you have two lists of information about people: one list has their names and phone numbers, and another has their names and email addresses. A natural join works like matching these lists by the common column "name" and combining the details into one list without repeating the name column twice.

In database terms, a natural join looks for all columns that share the same name in both tables and matches rows where these columns have the same values. It then merges these rows into one, removing duplicate columns. This makes it easy to combine related data without specifying the join condition explicitly.

Example

This example shows two tables: Students and Scores. Both have a column named student_id. A natural join combines these tables by matching student_id and merges their data.

sql

CREATE TABLE Students (
  student_id INT,
  name VARCHAR(50)
);

CREATE TABLE Scores (
  student_id INT,
  score INT
);

INSERT INTO Students VALUES (1, 'Alice'), (2, 'Bob');
INSERT INTO Scores VALUES (1, 85), (2, 90);

SELECT * FROM Students NATURAL JOIN Scores;

Output

student_id | name  | score
-----------|-------|-------
1          | Alice | 85
2          | Bob   | 90

When to Use

Use a natural join when you want to combine two tables that share one or more columns with the same names and you want to merge rows based on those columns automatically. It is helpful when the join condition is obvious and you want to write simpler queries without specifying the join keys.

For example, in a school database, you might join student information with their test scores using a natural join on student_id. This saves time and reduces errors by avoiding manual join conditions.

Key Points

• A natural join matches tables by all columns with the same names.
• It removes duplicate columns in the result.
• It simplifies queries by not requiring explicit join conditions.
• Be careful: if tables have unexpected common column names, results may be incorrect.

Key Takeaways

Natural join automatically matches and merges tables by columns with the same names.

It returns combined rows without repeating the common columns.

Use natural join for simple, clear relationships between tables sharing column names.

Always verify common column names to avoid unintended joins.

Natural join simplifies SQL queries by removing the need to specify join conditions.