JOINS in SQL

Table of Contents:

• What is JOIN in SQL?
• Why do we need JOIN in SQL?
• Types of JOINS in SQL
  • INNER JOIN in SQL
  • LEFT JOIN in SQL
  • RIGHT JOIN in SQL
  • FULL OUTER JOIN in SQL
• Using Multiple JOINS in a Single Query
• JOINS vs SUBQUERIES
• Common Mistakes and Solutions to Avoid Them
• Performance Comparison of JOINS in SQL
• Real-world Examples
• Best Practices
• Conclusion

Before getting started with the INNER JOIN, let us create some tables for better understanding.

Let’s create a Student table.

CREATE TABLE Students (
    student_id INT PRIMARY KEY,
    student_name VARCHAR(100),
    email VARCHAR(100)
);
INSERT INTO Students (student_id, student_name, email) VALUES
(1, 'John', '[email protected]'),
(2, 'Smith', '[email protected]'),
(3, 'Johnson', '[email protected]'),
(4, 'Wilson', '[email protected]');
Select * from Students;

CREATE TABLE Courses (
    course_id INT PRIMARY KEY,
    course_name VARCHAR(100),
    instructor VARCHAR(100)
);

INSERT INTO Courses (course_id, course_name, instructor) VALUES
(101, 'SQL for Beginners', 'Ramesh'),
(102, 'Advanced MySQL', 'Johnson'),
(103, 'Data Science', 'David'),
(104, 'Big Data', 'Antony');
Select * from Courses;

This is how the Courses table looks once it is created and inserted with the values.

Finally, let’s create an enrollment table

CREATE TABLE Enrollments (
    enrollment_id INT PRIMARY KEY,
    student_id INT,
    course_id INT,
    enrollment_date DATE,
    FOREIGN KEY (student_id) REFERENCES Students(student_id),
    FOREIGN KEY (course_id) REFERENCES Courses(course_id)
);
INSERT INTO Enrollments (enrollment_id, student_id, course_id, enrollment_date) VALUES
(1, 1, 101, '2024-01-15'),
(2, 1, 102, '2024-02-10'),
(3, 2, 103, '2024-02-20'),
(4, 3, 101, '2024-03-05'),
(5, 3, 104, '2024-03-10'),
(6, 4, 102, '2024-03-15');
Select * from Enrollments;

This is how the Enrollment table looks once it is created and inserted with the values.

What is JOIN in SQL?

A JOIN in SQL is used to combine two or more tables based on conditions or matching records between them.

Syntax:

SELECT column_names  
FROM table1  
JOIN table2  
ON table1.common_column = table2.common_column;

Why do we need JOIN in SQL?

• To maintain a consistent and accurate data retrieval: The use of primary and foreign keys provides a relationship between one table and another, while JOINS allow us to accurately and consistently retrieve our data between tables.
• Optimizing performance of queries: With a well-optimized JOIN (as well as indexes), it is faster to execute a JOIN than it would be as a programmatic process, fetching data and constructing a dataset.
• Retrieve Related Data: Data is stored in separate tables in relational databases to make the storage efficient and organized. 

Types of JOINS in SQL

There are different types of JOIN, like INNER, LEFT, RIGHT, and FULL OUTER JOIN.

INNER JOIN in SQL

INNER JOIN is a type of SQL join that retrieves records from two or more tables that have a matching value in the specified columns. It returns only those records where the joined records meet the specified join condition.

Syntax:

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

Example:

-- Using an INNER JOIN between the Student and Enrollment Tables
SELECT s.student_id, s.student_name, e.course_id
FROM Students s
INNER JOIN Enrollments e ON s.student_id = e.student_id;

Output:

Explanation: Here, the INNER JOIN matches student_id from the student’s table with student_id from the enrollment table. If a student is not enrolled in any of the courses, then they will not appear in the results.

LEFT JOIN in SQL

The LEFT JOIN in SQL retrieves all records from the left table and the matching records from the right table. If no match is found, NULL values are returned from the right table.

Syntax:

SELECT column_names
FROM table1
LEFT JOIN table2
ON table1.common_column = table2.common_column;

Example:

SELECT 
    Students.student_id,
    Students.student_name,
    Courses.course_name
FROM Students
LEFT JOIN Enrollments ON Students.student_id = Enrollments.student_id
LEFT JOIN Courses ON Enrollments.course_id = Courses.course_id;

Output:

Explanation: Here, all students, even those who are not enrolled in any courses, are returned by this query. Since every student is enrolled in this instance, there are no NULLs displayed. The course_name would display NULL if there were no enrolled students.

RIGHT JOIN in SQL

The RIGHT JOIN in SQL retrieves all the data from the right table and the matching data from the left table. If no match is found, NULL values are returned from the left table.

Syntax:

SELECT column_names
FROM table1
RIGHT JOIN table2
ON table1.common_column = table2.common_column

Example:

SELECT 
    Courses.course_id,
    Courses.course_name,
    Students.student_name
FROM Enrollments
RIGHT JOIN Courses ON Enrollments.course_id = Courses.course_id
LEFT JOIN Students ON Enrollments.student_id = Students.student_id;

Output:

Explanation: Here, even if none of the students are enrolled, this will show all courses. Courses with no enrollment will return NULL.

FULL OUTER JOIN in SQL

The FULL OUTER JOIN in SQL retrieves all the data from both tables, including the null values.

Syntax:

SELECT column_names
FROM table1
FULL OUTER JOIN table2
ON table1.common_column = table2.common_column;

Example:

-- LEFT JOIN: All students, even if not enrolled
SELECT 
    Students.student_name,
    Courses.course_name
FROM Students
LEFT JOIN Enrollments ON Students.student_id = Enrollments.student_id
LEFT JOIN Courses ON Enrollments.course_id = Courses.course_id

UNION

-- RIGHT JOIN: All courses, even if no student enrolled
SELECT 
    Students.student_name,
    Courses.course_name
FROM Courses
LEFT JOIN Enrollments ON Courses.course_id = Enrollments.course_id
LEFT JOIN Students ON Enrollments.student_id = Students.student_id;

Output:

Explanation: Even if some students are not enrolled, the first portion of the query retrieves all students and their enrolled courses. All courses, including those with no enrolled students, are retrieved in the second section of the query. Both results are combined using UNION to cover unmatched records from both sides, which is similar to a FULL OUTER JOIN.

NOTE: Since FULL OUTER JOIN is not supported in MySQL, the same output can be obtained from UNION.

Using Multiple JOINS in a Single Query

Multiple JOINS in a single query allow us to combine data from multiple related tables.

Syntax:

SELECT columns  
FROM table1  
JOIN table2 ON condition  
JOIN table3 ON condition;

Example:

SELECT 
    Students.student_name,
    Courses.course_name,
    Enrollments.enrollment_date
FROM Students
JOIN Enrollments ON Students.student_id = Enrollments.student_id
JOIN Courses ON Enrollments.course_id = Courses.course_id;

Output:

Explanation: Here, by using student_id, this query joins students to enrollments. Then, it uses course_id to join enrollments to courses. All three tables’ details are returned in a single result set.

JOINS vs SUBQUERIES

CREATE TABLE Employees (
    employee_id INT PRIMARY KEY,
    employee_name VARCHAR(50),
    department_id INT
);

CREATE TABLE Departments (
    department_id INT PRIMARY KEY,
    department_name VARCHAR(50)
);
INSERT INTO Employees VALUES (1, 'Alice', 101), (2, 'Bob', 102), (3, 'Charlie', 101);
INSERT INTO Departments VALUES (101, 'HR'), (102, 'IT'), (103, 'Finance');
SELECT e.employee_name, d.department_name  
FROM Employees e  
INNER JOIN Departments d ON e.department_id = d.department_id;

CREATE TABLE Customers (
    customer_id INT PRIMARY KEY,
    customer_name VARCHAR(50)
);
CREATE TABLE Orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    amount DECIMAL(10,2),
    FOREIGN KEY (customer_id) REFERENCES Customers(customer_id)
);
INSERT INTO Customers VALUES (1, 'John Doe'), (2, 'Jane Smith'), (3, 'Sam Wilson');
INSERT INTO Orders VALUES (101, 1, '2024-03-01', 250.50),  (102, 2, '2024-03-05', 100.00),  
(103, 1, '2024-03-10', 300.00);
SELECT c.customer_name, o.order_id, o.order_date, o.amount  
FROM Customers c  
INNER JOIN Orders o ON c.customer_id = o.customer_id;

Output:

Explanation: Here, only customers who have placed an order are retrieved by the INNER JOIN. Since Sam Wilson hasn’t placed any orders, he isn’t included in the result set.

Best Practices

• Use Indexing properly: To improve the performance, create an index on the columns.
• Choose the JOIN correctly: Based on the needs, choose the appropriate JOIN. For example, if you need to match the records, then use an INNER JOIN.
• Improve Performance: To improve the performance, avoid using SELECT * as it increases the data load and may impact performance.
• Filter Data: Before performing a JOIN, use the WHERE Clause, which reduces unnecessary processing.
• Ensure Readability: For better readability, prefer using an alias name like t1,t2, etc., to make the complex queries readable.

Conclusion

A JOIN in SQL is used to combine two or more tables based on conditions or matching records between them. As part of best practices, it is essential to avoid common mistakes such as inefficient use of JOINS, and optimisation for performance using filtering and indexing can reduce JOIN query time. In this blog, you have gained knowledge on various JOINs in detail.

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JOINS in SQL – FAQs