Difference between Inner Join and Equi Join and Natural Join

Inner Join vs Equi Join vs Natural Join: An Overview

SQL Join clause retrieves data from two or more database tables based on specific logical conditions or the connection between the tables. In this SQL Server tutorial, we are going to discuss the differences between Inner Join, Equi Join, and Natural Join.

If you think, you still aren't good with the Joins concepts, refer to Different Types of SQL Joins.

1. Inner Join

This is the most used join in the SQL. Inner Join is the by default join in MySQL. Hence, if you write JOIN it is the same as INNER JOIN. This join returns only those records/rows that match/exist in both the database tables.

How Does Inner Join Work?

This join returns only those records/rows that match/exist in both the database tables including the common column. From the above figure, we can clearly say that Inner Joins returns only the matching results from Table 1 and Table 2. This join joins two tables based on the column explicitly specified in the ON clause.

Read More: SQL Server Interview Questions and Answers

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Syntax of Inner Join

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name;

Inner Join Example

Below is a simple example showing the use of joins where the tables "tblDept" and "tblEmp" are being joined based on the DeptId column.

SELECT * FROM tblEmp JOIN tblDept
ON tblEmp.DeptID = tblDept.DeptID;

Inner Join Output

In the join condition, you can also use other operators like <,>,<>.

Read More: Basics of SQL Commands

Use Cases of Inner Join

• Combining Related Data: Inner join is used to retrieve data from multiple tables based on a related column or set of columns.
• Querying Related Entities: This is often used to query related entities in a relational database.
• Filtering Data: Inner join can filter data based on specific criteria.
• Performing Calculations: Inner join lets you perform calculations and aggregations across multiple related tables.
• Enforcing Referential Integrity: It does this by ensuring that only rows with matching values in the join columns are included in the result set.

2. Equi Join

Equi join is a special type of join also known as simple join in which we use only an equality operator("="). Hence, when you make a query for join using the equality operator, such a join query comes under Equi join.

How Does Equi Join Work?

The equi join creates a JOIN for equality or matching of the single or multiple column values of the relative tables. Apart from that, the equi join also creates the JOIN by using JOIN along with the ON clause and then providing the names of the columns with their relative tables to check equality using the equal operator.

Equijoin is a classified type of inner join that returns output by performing joining operations from two tables based on the common column that exists in them. The resultant result can have repeated column names.

Syntax of Equi Join

SELECT column_name (s)  
FROM table_name1, table_name2, ...., table_nameN  
WHERE table_name1.column_name = table_name2.column_name; 

OR

SELECT (column_list | *)  
FROM table_name1   
JOIN table_name2   
ON table_name1.column_name = table_name2.column_name;

Equi Join Example

Below is a simple example of equi having the use of ON condition.

SELECT * FROM tblEmp JOIN tblDept
ON tblEmp.DeptID = tblDept.DeptID;
--Using Clause is not supported by SQL Server
--Oracle and MySQL Query
SELECT * FROM tblEmp INNER JOIN tblDept USING(DeptID)

Equi Join Output

Note

1. Inner join can have equality (=) and other operators (like <,>,<>) in the join condition.
2. Equi join only has an equality (=) operator in the join condition.
3. Equi join can be an Inner join, Left Outer Join or Right Outer join.
4. The USING clause is not supported by SQL Server and Sybase. This clause is supported by Oracle and MySQL.

Use Cases of Equi Join

• Retrieving Related Data: Equi join is used to retrieve data from multiple tables having a common key or relationship.
• Querying Many-to-One Relationships: Equi join is ideal for querying many-to-one relationships between tables.
• Filtering Data Based on Relationships: Equi join allows you to filter data based on specific relationships between tables.
• Performing Joins Across Multiple Tables: Equi join can be extended to join more than two tables in a query.
• Calculating Aggregations: Equi join can be used to calculate aggregations across related tables.

3. Natural Join

A natural join in SQL is a type of equi join that occurs implicitly by comparing all the same name columns in both tables. The join result has only one column for each pair of equally named columns.

How Does Natural Join Work?

The Natural Join helps to join the two tables based on the same attribute name and the datatypes. The result of the JOIN table will contain all the attributes of both tables but keep only one copy of each common column accordingly. There is no need to specify the column names to join. An asterisk qualified by a table name (for example, Table_1.*) will be expanded to every column of that table that is not common.

Syntax of Natural Join

SELECT [column_names | *]   
FROM table_name1   
NATURAL JOIN table_name2;

Natural Join Example

Below is an example of a natural join. After using the Natual join, the result set will contain only one column for each pair of equally named columns. If there are no columns with the same names are found, then the result will be a "cross join".

--Run in Oracle and MySQL

SELECT * FROM tblEmp NATURAL JOIN tblDept

Natural Join Output

In the above join result, we have only one column "DeptID" for each pair of equally named columns.

Note

1. In Natural join, you can't see what columns from both the tables will be used in the join. In Natural Join, you might not get the desired result that you are expecting.
2. The natural join clause is not supported by SQL Server, it is supported by Oracle and MySQL.

References:http://support.microsoft.com/

Use Cases of Natural Join

• Joining Tables with Similar Structures: It simplifies the join operation by automatically matching columns based on their names.
• Retrieving Related Data: Natural join can be used to retrieve data from multiple tables that share common attributes.
• Querying Data Without Explicit Join Conditions: It automatically matches columns with the same name, making the query more concise and easier to write, especially for simple join operations.
• Reducing Query Complexity: Natural Join automatically determines the join conditions based on the column names, reducing the complexity of the query.
• Aggregating Data from Related Tables: Natural join can be used to aggregate data from related tables.

Difference between Natural Join, Equi Join, and Inner Join

What do you think?

I hope you will enjoy the tips while playing with SQL Server. If you want to gain a practical understanding, you can enroll in our SQL Server Course.

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