Left Join Versus Right Join

Introduction to SQL Joins

When working with databases, it’s common to encounter situations where you need to combine data from multiple tables. This is where SQL joins come in. A SQL join is used to combine rows from two or more tables based on a related column between them. There are several types of SQL joins, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN. In this blog post, we’ll focus on the difference between LEFT JOIN and RIGHT JOIN.

What is a Left Join?

A LEFT JOIN returns all the rows from the left table and the matched rows from the right table. If there are no matches, the result will contain NULL values for the right table. The syntax for a LEFT JOIN is as follows:

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

For example, suppose we have two tables, Customers and Orders, and we want to retrieve all customers and their corresponding orders.

Customers Table	Orders Table
CustomerID Name 1 John 2 Jane 3 Bob	OrderID CustomerID OrderDate 1 1 2020-01-01 2 1 2020-01-15 3 2 2020-02-01

The result of the LEFT JOIN would be:

CustomerID	Name	OrderID	CustomerID	OrderDate
1	John	1	1	2020-01-01
1	John	2	1	2020-01-15
2	Jane	3	2	2020-02-01
3	Bob	NULL	NULL	NULL

What is a Right Join?

A RIGHT JOIN returns all the rows from the right table and the matched rows from the left table. If there are no matches, the result will contain NULL values for the left table. The syntax for a RIGHT JOIN is as follows:

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

Using the same example as above, the result of the RIGHT JOIN would be:

CustomerID	Name	OrderID	CustomerID	OrderDate
1	John	1	1	2020-01-01
1	John	2	1	2020-01-15
2	Jane	3	2	2020-02-01

Note that the row for Bob is not included in the result because there is no matching order for him in the Orders table.

📝 Note: The main difference between a LEFT JOIN and a RIGHT JOIN is the order of the tables in the join operation. A LEFT JOIN returns all rows from the left table, while a RIGHT JOIN returns all rows from the right table.

When to Use Left Join and Right Join

Here are some scenarios where you might use LEFT JOIN and RIGHT JOIN: * Use LEFT JOIN when you want to retrieve all rows from the left table and the matching rows from the right table. * Use RIGHT JOIN when you want to retrieve all rows from the right table and the matching rows from the left table. * Use LEFT JOIN when you want to find all customers who have placed an order, as well as those who have not placed an order. * Use RIGHT JOIN when you want to find all orders that have been placed, as well as the customers who placed them.

Best Practices for Using Left Join and Right Join

Here are some best practices to keep in mind when using LEFT JOIN and RIGHT JOIN: * Always specify the join condition using the ON keyword. * Use table aliases to simplify the query and improve readability. * Avoid using RIGHT JOIN whenever possible, as it can be confusing and may lead to incorrect results. * Use LEFT JOIN instead of RIGHT JOIN to retrieve all rows from the left table and the matching rows from the right table.

In summary, LEFT JOIN and RIGHT JOIN are two types of SQL joins that can be used to combine rows from two or more tables based on a related column. While they are similar, they have some key differences in terms of the order of the tables and the result set. By following best practices and using the correct join type, you can write efficient and effective SQL queries to retrieve the data you need.