Joins in DBMS

Last Updated on December 27, 2023 by Ankit Kochar

Welcome to the comprehensive guide on Joins in Database Management Systems (DBMS). Joins are fundamental operations in DBMS that allow the combination of data from multiple tables based on a related column between them. They play a crucial role in querying databases efficiently and retrieving meaningful insights from interconnected data.

In this article, we’ll delve into the various types of joins, their functionalities, syntax, and common use cases. Whether you’re a beginner looking to understand the basics or an experienced professional seeking a refresher, this guide aims to provide you with a solid understanding of joins in DBMS.

What are Joins in DBMS?

A join is a way to combine data from two or more tables based on a common column. For example, let’s say we have two tables: Customers and Orders. The Customers table contains information about each customer, including their name, address, and email address. The Orders table contains information about each order, including the order date, product name, and quantity.

To combine data from these two tables, we can join them on the customer ID column, which is common to both tables. By doing so, we can retrieve information about each customer’s orders in a single query.

Types of Joins in DBMS

There are several types of joins in DBMS, each with its own syntax and use case. The following are the different types of Joins in DBMS.

• Inner Join
  • Theta Join
  • Equi Join
• Natural Join
• Outer Join
  • Left Outer Join
  • Right Outer Join
  • Full Outer Join

We will understand all of these joins in DBMS with the help of the following tables.

Table1:

ID	Name	Age
1	Alice	23
2	Bob	28
3	Charlie	32

Table2:

ID	Address	Salary
2	New York	50000
3	Boston	65000
4	San Diego	75000

Inner Join

An Inner Join returns only the rows in both tables that match the join condition.

Theta Join
A Theta Join uses a condition other than equality to join two tables. It uses operators like , =, , etc., to define the join condition.

Syntax of Theta Join

SELECT table1.column1, table2.column2
FROM table1
INNER JOIN table2
ON table1.columnX operator table2.columnY;

Example of Theta Join
Consider the above two tables.

Query:
To perform a Theta Join, we can join the two tables on the condition that the age in Table1 is greater than the salary in Table2.

SELECT Table1.Name, Table2.Address
FROM Table1
INNER JOIN Table2
ON Table1.Age > Table2.Salary;

Output:
This query would return the following result:

Name	Address

Here we get an empty table as a result because not a single entry is following the condition specified in the join

Equi Join
An Equi Join returns all the rows in both tables where the specified
columns are equal.

Syntax of Equi Join

SELECT table1.column1, table2.column2
FROM table1
INNER JOIN table2
ON table1.columnX = table2.columnY;

Example of Equi Join
Taking reference from the above tables, table 1 and table 2.

Query:
To perform an Equi Join, we can join the two tables on the ID column.

SELECT Table1.Name, Table2.Address
FROM Table1
INNER JOIN Table2
ON Table1.ID = Table2.ID;

Output:
This query would return the following result:

Name	Address
Bob	New York
Charlie	Boston

Natural Join

A Natural Join is a type of Join that matches columns with the same name in both tables.

Syntax of Natural Join

SELECT table1.column1, table2.column2
FROM table1
NATURAL JOIN table2;

Example of Natural Join
Consider the above two tables:

Query:
To perform a Natural Join, we can simply use the following query:

SELECT Table1.ID, Table1.Name, Table1.Age, Table2.Address, Table2.Salary
FROM Table1
NATURAL JOIN Table2;

Output:
This query would return the following result:

ID	Name	Age	Address	Salary
2	Bob	28	New York	50000
3	Charlie	32	Boston	65000

Outer Join

An Outer Join in DBMS returns all the rows from one table and the matching rows from the other table. If there is no match, NULL values are returned for the missing rows.

Left Outer Join
A Left Outer Join in DBMS returns all the rows from the left table and the matching rows from the right table. If there is no match, NULL values are returned for the missing rows.

Syntax of Left Outer Join

SELECT table1.column1, table2.column2
FROM table1
LEFT JOIN table2
ON table1.columnX = table2.columnY;

Example of Left Outer Join
Again Considering the above two tables:

Query:
To perform a Left Outer Join, we can join the two tables on the ID column.

SELECT Table1.Name, Table2.Address
FROM Table1
LEFT JOIN Table2
ON Table1.ID = Table2.ID;

Output:
This query would return the following result:

Name	Address
Alice	NULL
Bob	New York
Charlie	Boston

Right Outer Join
A Right Outer Join returns all the rows from the right table and the matching rows from the left table. If there is no match, NULL values are returned for the missing rows.

Syntax of Right Outer Join

SELECT table1.column1, table2.column2
FROM table1
RIGHT JOIN table2
ON table1.columnX = table2.columnY;

Example of Right Outer Join
Consider the above two tables:

Query:
To perform a Right Outer Join, we can join the two tables on the ID column.

SELECT Table1.Name, Table2.Address
FROM Table1
RIGHT JOIN Table2
ON Table1.ID = Table2.ID;

Output:
This query would return the following result:

Name	Address
Bob	New York
Charlie	Boston
NULL	San Diego

Full Outer Join
A Full Outer Join returns all the rows from both tables and NULL values for the missing rows.

Syntax of Full Outer Join

SELECT table1.column1, table2.column2
FROM table1
FULL OUTER JOIN table2
ON table1.columnX = table2.columnY;

Example of Full Outer Join
Considering the above-mentioned two tables i.e., Table1 and Table2:

Query:
To perform a Full Outer Join, we can join the two tables on the ID column.

SELECT Table1.Name, Table2.Address
FROM Table1
FULL OUTER JOIN Table2
ON Table1.ID = Table2.ID;

Output:
This query would return the following result:

Name	Address
Alice	NULL
Bob	New York
Charlie	Boston
NULL	San Diego

A Quick Revision of Joins in DBMS

To assist you in a good revision, a quick revision on Joins in DBMS is given below:

• Joins in DBMS is used to combine tables.
• There are three types of joins: inner joins, natural joins, and outer joins.
• Inner joins are classified into two types: Theta Join(for relational operators) and Equi Join(for Equality).
• There are three types of outer joins in DBMS: left outer join, right outer join, and full outer join.
• Natural join is only performed when at least one matching attribute exists in both tables.
• No matter the Join condition, a left outer join always returns every row from the left table.
• Regardless of the Join condition, Right Outer Join always returns all rows from the right table.
• Regardless of the join condition, Complete Outer Join always returns all rows from both tables.

Conclusion
In conclusion, joins are an indispensable aspect of working with relational databases. They enable the amalgamation of data spread across different tables, facilitating the extraction of valuable information and insights. By comprehending the types of joins available—such as INNER JOIN, LEFT JOIN, RIGHT JOIN, and OUTER JOIN—and their applications, database administrators and developers can optimize query performance, ensure data accuracy, and derive meaningful results.

Remember, mastering the art of utilizing joins effectively involves a combination of theoretical knowledge and practical application. With continuous practice and exploration, one can leverage joins efficiently to handle complex data scenarios in DBMS, thereby enhancing the efficiency and productivity of database-related tasks

FAQs Related to Joins in DBMS

Some Frequently Asked Questions on Joins in DBMS are given below.

Q1. What is the difference between Inner Join and Outer Join?
The main difference between Inner Join and Outer Join is that Inner Join returns only the matching records from both tables, while Outer Join returns all records from one table and matching records from the other table.

Q2: What are the different types of joins in DBMS?
The common types of joins in DBMS include:

• INNER JOIN: Retrieves records that have matching values in both tables.
• LEFT JOIN: Retrieves all records from the left table and matching records from the right table.
• RIGHT JOIN: Retrieves all records from the right table and matching records from the left table.
• OUTER JOIN: Retrieves all records when there is a match in either the left or right table.

Q3: How do joins improve query performance in databases?
Joins optimize query performance by minimizing data redundancy, allowing for efficient data retrieval, and facilitating the extraction of specific information from multiple related tables rather than querying each table separately.

Q4: What are some common use cases for joins in DBMS?
Joins are commonly used for tasks such as generating reports, analyzing data from multiple sources, retrieving information for business intelligence, and creating complex data views to support decision-making processes.

Q5: Are there any limitations or challenges associated with using joins in DBMS?
While joins are powerful, they can lead to performance issues with large datasets or complex queries. Additionally, improper usage or excessive joins in a query can result in slower execution times and increased resource consumption. It’s crucial to optimize queries and understand the database schema to mitigate these challenges.