What is Natural Join in SQL - A Comprehensive Guide

What Is A Natural Join In SQL?

A natural join is a type of equijoin, which means it works by matching equal values in common columns. Unlike other joins, we do not need to explicitly specify the column for joining, as SQL automatically takes care of finding the common column names and data types and joins the data.

The key features of a natural join are:

• It is an INNER JOIN by default that returns only matching rows between the tables.
• Tables are joined based on all columns with the same name and data types. The SQL engine automatically detects these common columns.
• Output contains unique columns; common join columns appear only once.
• ON or USING clauses cannot be used to specify join columns, as join uses all common column names implicitly.
• Can be combined with left, right, and full outer joins for more flexibility.

Syntax

The syntax for a natural join in SQL is fairly simple and straightforward. It brings together two or more tables by just using the NATURAL JOIN keywords without needing to specify the actual join columns with the ON or USING clause. The basic syntax is:

SELECT column1, column2, column3,...columnN

FROM tablename1

NATURAL JOIN tablename2

Here, columns 1 to N are the names of the columns that you want to retrieve data from in the result set after the join operation. The first table name specifies the left table, and the second tablename is the right table for the join. The NATURAL JOIN keywords inform the SQL engine to automatically join these tables on columns with the same name and data type.

Steps To Implement SQL Natural Join

Performing a natural join in SQL involves just three simple steps:

1. Identify Common Column Names: Examine the tables to be joined and determine if there are one or more column names that are common between them. For a natural join, even the data types of these columns should match.
2. Write SQL Query: Write a basic SELECT query by specifying the required columns after SELECT and mentioning table names after the FROM clause. Place the NATURAL JOIN keywords between the table names.
3. Execute Query: Run the SQL query to perform the natural join on common column names between the specified tables and retrieve rows combined from the tables into a result set.

Following these 3 steps and using the built-in NATURAL JOIN feature allows for easily joining SQL tables without explicitly stating the join columns.

Example Of Natural Join In SQL

Consider the following student and fee tables with common column names and data types; here’s a natural join SQL example:

Student

Roll No. Name     Contact

1               John   1234567890

2               Sam   0987654321

Fee

Roll No. Amount Paid

1               5000               Yes

2               4000                   No

The SQL query for a natural join:

SELECT s.Name, s.Contact, f.Amount, f.Paid

FROM Student s

NATURAL JOIN Fee f;

Output:

Name Contact Amount Paid

John 1234567890     5000 Yes

Sam 0987654321        4000       No

The tables were joined based on the common RollNo column.

Difference Between Natural Join And Inner Join

There are some key differences between SQL natural joins and inner joins:

• Natural joins automatically join tables based on column names, while inner joins require explicitly specifying the join columns in the ON or USING clause.
• Natural join returns only one instance of a common join column, while inner join returns duplicate join columns from both tables.
• Additional joins like LEFT, RIGHT, etc. can be combined with natural join but usually not with inner join.
• Natural join provides simpler, automated joining of tables, while inner join gives more control by specifying the exact join conditions.

So, in essence, natural joins are easier to implement, while inner joins are more flexible and customizable during the table joining process in SQL.

Conclusion

Natural join in SQL provides a simpler and automated method for combining related data from tables without needing to specify the join columns explicitly. It is easier to implement but needs caution to avoid accidental joins. Understanding natural joins is key to effectively using SQL for data analysis tasks.

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FAQs

1. What Is A Natural Join With No Common Attributes?

A natural join on tables with no common columns results in a cartesian product where each row from one table is joined with every row of the other table. This is rarely useful.

2. Can I Use Natural Join With Multiple Tables?

Yes, a natural join can be implemented across multiple tables as long as they have one or more common column names and data types.

3. Are Natural Joins Suitable For Large Datasets?

Natural joins can create performance issues with very large datasets, as SQL needs to scan all columns to find common names. In such cases, it is better to use inner join by specifying join columns.

4. How Can I Avoid Ambiguity In Natural Joins?

Give table aliases before column names in Select and ensure column names are unique across tables to avoid ambiguity.

5. Are Natural Joins Supported In All SQL Database Systems?

Most database systems, including MySQL, SQL Server, Oracle, etc., support natural joins, but syntax may differ slightly across systems.