SQL Joins (Inner, Left, Right and Full Join)

SQL joins are fundamental tools for combining data from multiple tables in relational databases. Joins allow efficient data retrieval, which is essential for generating meaningful observations and solving complex business queries. Understanding SQL join types, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, and NATURAL JOIN, is critical for working with relational databases.

In this article, we will cover the different types of SQL joins, including INNER JOIN, LEFT OUTER JOIN, RIGHT JOIN, FULL JOIN, and NATURAL JOIN. Each join type will be explained with examples, syntax, and practical use cases to help us understand when and how to use these joins effectively.

What is SQL Join?

An SQL JOIN clause is used to query and access data from multiple tables by establishing logical relationships between them. It can access data from multiple tables simultaneously using common key values shared across different tables. We can use SQL JOIN with multiple tables. It can also be paired with other clauses, the most popular use will be using JOIN with WHERE clause to filter data retrieval.

Before diving into the specifics, let’s visualize how each join type operates:

• INNER JOIN: Returns only the rows where there is a match in both tables.
• LEFT JOIN (LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table. If there’s no match, NULL values are returned for columns from the right table.
• RIGHT JOIN (RIGHT OUTER JOIN): Returns all rows from the right table, and the matched rows from the left table. If there’s no match, NULL values are returned for columns from the left table.
• FULL JOIN (FULL OUTER JOIN): Returns all rows when there is a match in one of the tables. If there’s no match, NULL values are returned for columns from the table without a match.

1. SQL INNER JOIN

The INNER JOIN keyword selects all rows from both the tables as long as the condition is satisfied. This keyword will create the result-set by combining all rows from both the tables where the condition satisfies i.e value of the common field will be the same. 

Syntax

SELECT table1.column1,table1.column2,table2.column1,…. FROM table1 INNER JOIN table2 ON table1.matching_column = table2.matching_column;

Note: We can also write JOIN instead of INNER JOIN. JOIN is same as INNER JOIN. 

Example of INNER JOIN

Consider the two tables, Student and StudentCourse, which share a common column ROLL_NO. Using SQL JOINS, we can combine data from these tables based on their relationship, allowing us to retrieve meaningful information like student details along with their enrolled courses. 

Student Table

StudentCourse Table

Let’s look at the example of INNER JOIN clause, and understand it’s working. This query will show the names and age of students enrolled in different courses.  

Query:

SELECT StudentCourse.COURSE_ID, Student.NAME, Student.AGE FROM Student
INNER JOIN StudentCourse
ON Student.ROLL_NO = StudentCourse.ROLL_NO;

Output

2. SQL LEFT JOIN

A LEFT JOIN returns all rows from the left table, along with matching rows from the right table. If there is no match, NULL values are returned for columns from the right table. LEFT JOIN is also known as LEFT OUTER JOIN.

Syntax

SELECT table1.column1,table1.column2,table2.column1,….
FROM table1
LEFT JOIN table2
ON table1.matching_column = table2.matching_column;

Note: We can also use LEFT OUTER JOIN instead of LEFT JOIN, both are the same.

LEFT JOIN Example

In this example, the LEFT JOIN retrieves all rows from the Student table and the matching rows from the StudentCourse table based on the ROLL_NO column.

Query:

SELECT Student.NAME,StudentCourse.COURSE_ID 
FROM Student
LEFT JOIN StudentCourse 
ON StudentCourse.ROLL_NO = Student.ROLL_NO;

Output

3. SQL RIGHT JOIN

RIGHT JOIN returns all the rows of the table on the right side of the join and matching rows for the table on the left side of the join. It is very similar to LEFT JOIN for the rows for which there is no matching row on the left side, the result-set will contain null. RIGHT JOIN is also known as RIGHT OUTER JOIN. 

Syntax

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

Key Terms

• table1: First table.
• table2: Second table
• matching_column: Column common to both the tables.

Note: We can also use RIGHT OUTER JOIN instead of RIGHT JOIN, both are the same. 

RIGHT JOIN Example

In this example, the RIGHT JOIN retrieves all rows from the StudentCourse table and the matching rows from the Student table based on the ROLL_NO column.

Query:

SELECT Student.NAME,StudentCourse.COURSE_ID 
FROM Student
RIGHT JOIN StudentCourse 
ON StudentCourse.ROLL_NO = Student.ROLL_NO;

Output

4. SQL FULL JOIN

FULL JOIN creates the result-set by combining results of both LEFT JOIN and RIGHT JOIN. The result-set will contain all the rows from both tables. For the rows for which there is no matching, the result-set will contain NULL values.

Syntax

SELECT table1.column1,table1.column2,table2.column1,....
FROM table1 
FULL JOIN table2
ON table1.matching_column = table2.matching_column; 

Key Terms

• table1: First table.
• table2: Second table
• matching_column: Column common to both the tables.

FULL JOIN Example

This example demonstrates the use of a FULL JOIN, which combines the results of both LEFT JOIN and RIGHT JOIN. The query retrieves all rows from the Student and StudentCourse tables. If a record in one table does not have a matching record in the other table, the result set will include that record with NULL values for the missing fields

Query:

SELECT Student.NAME,StudentCourse.COURSE_ID 
FROM Student
FULL JOIN StudentCourse 
ON StudentCourse.ROLL_NO = Student.ROLL_NO;

Output 

5. SQL Natural Join (?)

Natural join can join tables based on the common columns in the tables being joined. A natural join returns all rows by matching values in common columns having same name and data type of columns and that column should be present in both tables.

• Both table must have at least one common column with same column name and same data type.
• The two table are joined using Cross join.
• DBMS will look for a common column with same name and data type. Tuples having exactly same values in common columns are kept in result.

Natural join Example

Look at the two tables below- Employee and Department

Problem: Find all Employees and their respective departments.

Solution Query: (Employee) ? (Department)

Conclusion

SQL joins are essential tools for anyone working with relational databases. Understanding the different types of joins in SQL, like INNER JOIN, LEFT OUTER JOIN, RIGHT JOIN, and FULL JOIN, allows us to combine and query data effectively. With the examples and syntax covered here, we should feel confident applying these SQL join types to our data to retrieve meaningful observations and manage complex queries with ease. Use these SQL join techniques to streamline our data handling and enhance our SQL skills.