Understand the difference between Left Join and Left Outer Join

In the world of databases and SQL, the LEFT JOIN and LEFT OUTER JOIN are two commonly used operations for joining tables together. While these two terms are often used interchangeably, it's essential to understand the subtle differences, especially when it comes to specific database dialects like MySQL.

In this blog post, you will learn the subtle nuances about these two.

What is a LEFT JOIN

A LEFT JOIN is a type of join operation in SQL that combines rows from two tables based on a related column between them. The result includes all the rows from the left table, regardless of whether there are matching rows in the right table. If there is no match for a row in the right table, NULL values are used for the columns from the right table.

Example of LEFT JOIN

SELECT *
FROM table1
LEFT JOIN table2 ON table1.id = table2.id;

What is a LEFT OUTER JOIN?

On the other hand, a LEFT OUTER JOIN is another way to specify a left join operation in SQL. However, in standard SQL, the OUTER keyword is optional and can be omitted. The LEFT OUTER JOIN includes all the rows from the left table, just like the LEFT JOIN.

Example of LEFT OUTER JOIN

SELECT *
FROM table1
LEFT OUTER JOIN table2 ON table1.id = table2.id;

Understanding the Difference

The crucial point to note is that in most database systems, such as MySQL, LEFT JOIN and LEFT OUTER JOIN are synonymous and can be used interchangeably. Both operations perform a left outer join, where all the rows from the left table are included in the result set.

While the syntax may vary slightly in different database dialects, the intent and the outcome of using LEFT JOIN and LEFT OUTER JOIN remain the same. It's essential to be aware of these nuances, particularly when working with specific databases that might require one syntax over the other.

In conclusion

The choice between using LEFT JOIN and LEFT OUTER JOIN in SQL depends on personal preference and the specific requirements of the database system you are using. Regardless of the terminology used, both operations achieve the same result – including all rows from the left table in the output of a join operation.