What are SQL JOINs: Inner, Outer, and Cross JOINs with Examples

SQL, or Structured Query Language, is regarded as the core of relational databases, which provides aid in handling data. It offers extensive features, including Injection, Triggers, Joins, Hosting, etc.

Joins are one of the most important concepts of SQL. In this article, we will give you informative insights into SQL JOINs and also look into the different types of SQL Joins. Please keep reading until the end of the article to know more about SQL Joins in detail. Let’s begin!

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What is SQL?

SQL is a programming language that is used for storing and processing information within a relational database. A relational database stores information in a tabular format representing different data attributes and relationships between the data values.

You can also use SQL statements for storing, updating, removing, searching, and retrieving information from the database. Besides, it can also be used for maintaining and optimising database performance.

Are you preparing yourself for SQL? If so, then go through these SQL-interview-questions.

What are JOINs in SQL?

JOINs in SQL are typical commands that combine rows from different tables based on related columns between those tables. Users predominantly use this command while extracting data from tables that share many-to-many relationships.

Importance of SQL JOINs

SQL JOINs enable you to combine data from tables based on related columns. The importance of SQL JOINs can be stated as follows:

• Data integrity
• Data retrieval
• Optimised performance
• Flexible data query
• Business intelligence
• Query optimisation, etc.

Different Types of SQL JOINs

The SQL server primarily supports four distinctive variants of JOINs. Each JOIN query in SQL defines how two or more tables are related within a query. 

Here we have enlisted the various types of SQL JOIN support within the SQL Server.

• Inner Joins in SQL
• Outer Joins in SQL
• Cross Joins in SQL
• Right Joins in SQL
• Left Joins in SQL
• Full-outer Joins in SQL

Inner JOINs in SQL

This type of JOIN in SQL returns all records from multiple tables that satisfy the specified JOIN condition. It is considered the simplest and the most popular form of JOIN in SQL that assumes a default JOIN. Omitting the INNER keyword from the JOIN query will fetch you a similar output.

Functions of Inner JOINs in SQL

This type of JOIN in SQL combines rows from two or more tables based on an entirely matching condition. It only returns the rows that have matching values in both tables.

Syntax and How to Use Them

SELECT columns    
FROM table1    
INNER JOIN table2 ON condition1    
INNER JOIN table3 ON condition2    

Examples

Firstly, we will create two tables “Student” and “Fee” using the following statement:
CREATE TABLE Student (      
  id int PRIMARY KEY IDENTITY,     
  admission_no varchar(45) NOT NULL,  
  first_name varchar(45) NOT NULL,      
  last_name varchar(45) NOT NULL,  
  age int,  
  city varchar(25) NOT NULL      
);    
  
CREATE TABLE Fee (   
  admission_no varchar(45) NOT NULL,  
  course varchar(45) NOT NULL,      
  amount_paid int,    
);
Now, we will incorporate a few records into these tables by using these statements:
INSERT INTO Student (admission_no, first_name, last_name, age, city)
VALUES (3354,'Luisa', 'Evans', 13, 'Texas'),       
(2135, 'Paul', 'Ward', 15, 'Alaska'),       
(4321, 'Peter', 'Bennett', 14, 'California'),    
(4213,'Carlos', 'Patterson', 17, 'New York'),       
(5112, 'Rose', 'Huges', 16, 'Florida'),  
(6113, 'Marielia', 'Simmons', 15, 'Arizona'),    
(7555,'Antonio', 'Butler', 14, 'New York'),       
(8345, 'Diego', 'Cox', 13, 'California');

INSERT INTO Fee (admission_no, course, amount_paid)       
VALUES (3354,'Java', 20000),       
(7555, 'Android', 22000),       
(4321, 'Python', 18000),    
(8345,'SQL', 15000),       
(5112, 'Machine Learning', 30000);

Next, you need to execute the SELECT statement for verifying the records:

Table: Student

Table: Fee

Best Practices and Common Pitfalls 

• Using proper indexing

• Using aliases for readability
• Avoiding excessive tables, etc.

Some of the common pitfalls that you’re likely to encounter while using this JOIN in SQL are:

• Incorrect JOIN conditions
• Data skewness
• Performance issues, etc.

Outer JOINs

Outer JOINs in SQL Server return all records from both tables that genuinely satisfies the JOIN condition. Simply put, thisSQL  JOIN will return the matching record and all the unmatched rows from all tables.

Explanation of Outer JOINs and Their Function

Outer JOINs in SQL return all rows from one table and matching rows from another. Furthermore, it also includes non-matching rows from one or both tables.

Syntax for Left, Right, and Full Outer SQL JOINs and How to Use Them

The following syntax illustrates the use of left outer JOIN in SQL Server:

SELECT column_lists  
FROM table1    
LEFT [OUTER] JOIN table2    
ON table1.column = table2.column;
The following syntax illustrates the use of right outer JOIN in SQL Server:
SELECT column_lists    
FROM table1    
RIGHT [OUTER] JOIN table2    
ON table1.column = table2.column;
The following syntax illustrates the use of full outer JOIN in SQL Server:
SELECT column_lists    
FROM table1    
FULL [OUTER] JOIN table2    
ON table1.column = table2.column;

Examples of Outer JOINs in Action

We can seamlessly demonstrate the LEFT OUTER JOIN using the following command:

SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student  
LEFT OUTER JOIN Fee  
ON Student.admission_no = Fee.admission_no;
Here’s howe you can use the RIGHT OUTER JOIN to get records from both tables:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student  
RIGHT OUTER JOIN Fee  
ON Student.admission_no = Fee.admission_no;
This example clearly explains the techniques of using the FULL OUTER JOIN to get records from both tables:
SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student  
FULL OUTER JOIN Fee  
ON Student.admission_no = Fee.admission_no;

Best Practices and Common Pitfalls When Using Outer JOINs

Some of the best practices for using Outer JOINs include:

• Using appropriate JOIN syntax
• Defining the primary purpose of JOIN
• Understanding the order of operations
• Testing the results with sample data

Some of the most common pitfalls that you’re likely to encounter are:

• Using inappropriate JOIN syntax
• Understanding the differences between various types of outer JOINs
• Not accounting for NULL values

Cross JOINs

The cross JOINs in SQL combine all the possibilities of different tables, returning results from each row of every contributing table. 

Explanation of Cross JOINs and Their Function

Cross JOIN is a join in SQL that produces a Cartesian product of two tables. It results in all possible combinations of the rows and doesn’t require any standard columns between the tables.

Syntax for Cross JOINs in SQL  and How to Use Them

SELECT column_lists    
FROM table1    
CROSS JOIN table2;

Examples of Cross JOINs in Action

Here’s how we can demonstrate the CROSS JOIN with the following command:

SELECT Student.admission_no, Student.first_name, Student.last_name, Fee.course, Fee.amount_paid
FROM Student  
CROSS JOIN Fee  
WHERE Student.admission_no = Fee.admission_no;

Best Practices and Common Pitfalls When Using Cross JOINs

The best practices for using cross JOINs include:

• Using proper filtering only to limit the size of the result set, understanding data and the primary purpose of SQL JOIN
• Testing all results with the help of sample data

Some of the common pitfalls that you might encounter are:

• An extensive number of rows
• Producing unintended combinations
• Noticeable performance issues

Comparison of Different Types of JOINs in SQL

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Pros and Cons of Each Type of JOINs in SQL

Here we have listed down the pros and cons of each type of JOIN query in SQL:

INNER JOIN:

Pros:

• Returns only the matching rows, which can improve query performance.
• Easy to understand and use.

Cons:

• Can exclude non-matching rows that might be relevant to the query.

Check out more on Inner JOIN SQL.

LEFT JOIN

Pros:

• Returns all rows from the left table, including non-matching rows.
• Useful for finding all records in one table that match or don't match records in another table.

Cons:

• It can produce NULL values, which might require additional handling.
• Slower than INNER JOIN due to the larger result set.

RIGHT JOIN

Pros:

• Returns all rows from the right table, including non-matching rows.
• Useful for finding all records in one table that match or don't match records in another table.

Cons:

• It can produce NULL values, which might require additional handling.
• Slower than INNER JOIN due to the more significant result set.

FULL OUTER JOIN

Pros:

• Returns all rows from both tables, including non-matching rows.
• Useful for finding all records that exist in one or both tables.

Cons:

• It can produce a substantial result set.
• It can be slow due to the more significant result set.

CROSS JOIN

Pros:

• It can help generate all possible combinations of rows from both tables.
• Useful for testing and generating data sets.

Cons:

• It can produce a huge result set.
• Not suitable for most queries due to the large number of combinations generated.

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Choosing the Right Type of SQL JOIN for Your Data Needs

Consider the relationship between the data and tables you wish to retrieve to pick out the right JOIN type. You can use the INNER JOIN to retrieve matching data and LEFT or RIGHT JOIN to include non-matching data. Besides, you can also use the FULL OUTER JOIN to include all data and CROSS JOIN to combine all possible combinations.

Conclusion

In this article we have what SQL joins and, the major types of joins in SQL. To sum up, SQL JOINs enable you to combine data from multiple tables. You must choose the appropriate SQL JOIN type based on the relationship between tables and data requirements.

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