Left join and left outer join in SQL: Differences

When writing SQL queries that involve combining data from multiple tables, one of the most commonly used operations is the JOIN. Particularly, the LEFT JOIN (also known as the LEFT OUTER JOIN) is a powerful tool used to retrieve all records from one table while pulling in related records from another. These terms—LEFT JOIN and LEFT OUTER JOIN—are often used interchangeably, leading to confusion for those new to SQL. Are they truly the same, or do subtle differences separate them?

TL;DR

LEFT JOIN and LEFT OUTER JOIN are functionally identical in SQL. The word “OUTER” is optional and used for clarity or stylistic preference. They both return all records from the left table and the matched rows from the right table, inserting NULLs when there’s no match. The key difference lies in naming conventions, not results or performance.

What is a LEFT JOIN?

A LEFT JOIN is a type of SQL join that returns all the records from the left table (the first table listed in the join clause), and the matched records from the right table. If there are no matches from the right table, the result will still include the left table’s row with NULLs for the columns from the right table.

This kind of join is particularly useful in scenarios where you want to preserve every entry from one table regardless of whether a matching record exists in the related table.

Syntax Example

SELECT employees.name, departments.dept_name
FROM employees
LEFT JOIN departments ON employees.dept_id = departments.dept_id;

This query retrieves all employees and their corresponding department names. If an employee isn’t assigned to a department, the dept_name will be NULL.

What is a LEFT OUTER JOIN?

LEFT OUTER JOIN is another way of expressing a LEFT JOIN. In SQL, “OUTER” is optional and simply adds clarity to the type of join being performed. The word “OUTER” may help some developers quickly understand the scope of the join, especially when paired with other join types like RIGHT OUTER JOIN or FULL OUTER JOIN.

Same Syntax, Just More Explicit

SELECT employees.name, departments.dept_name
FROM employees
LEFT OUTER JOIN departments ON employees.dept_id = departments.dept_id;

This query is exactly the same as the one shown above using LEFT JOIN. The output will be identical in terms of structure, number of records returned, and the values in those records.

Are LEFT JOIN and LEFT OUTER JOIN Different?

The short answer is: No. There is no difference in how a LEFT JOIN and a LEFT OUTER JOIN operate. They are two ways of expressing the same SQL operation. The inclusion of the word “OUTER” is purely a matter of syntax and style.

Understanding SQL Join Types

To fully appreciate the role of LEFT JOINs, it helps to understand the wider family of SQL joins. These are:

• INNER JOIN: Returns only matched rows from both tables.
• LEFT JOIN or LEFT OUTER JOIN: Returns all rows from the left table, and matched rows from the right table. Unmatched rows get NULLs.
• RIGHT JOIN or RIGHT OUTER JOIN: Returns all rows from the right table and matched rows from the left.
• FULL OUTER JOIN: Returns matched rows from both sides, as well as unmatched rows from both, filling NULLs where needed.

Why Have Two Names for the Same Operation?

The reason lies in SQL’s roots and standards. SQL is governed by ANSI (American National Standards Institute), and according to the ANSI SQL standard, the correct and complete syntax for an outer join is to specify “OUTER”. Accordingly:

• LEFT OUTER JOIN
• RIGHT OUTER JOIN
• FULL OUTER JOIN

However, for the sake of brevity and simplicity, most modern SQL engines—including MySQL, PostgreSQL, SQL Server, and Oracle—allow you to drop the word “OUTER”. So, “LEFT JOIN” is understood to mean “LEFT OUTER JOIN”.

This shorthand has become so widely adopted that “LEFT JOIN” is far more commonly seen in codebases and tutorials than the full version.

Practical Scenarios Where LEFT JOIN is Useful

Let’s consider a few common use cases where LEFT JOINs shine:

1. Reporting on Orphan Records

If you want to identify entries in one table that don’t exist in another, like users who’ve never placed an order:

SELECT users.username, orders.order_id
FROM users
LEFT JOIN orders ON users.user_id = orders.user_id
WHERE orders.order_id IS NULL;

2. Maintaining Data Completeness

When generating reports, you may need a comprehensive view that includes all departments—even those with no current employees:

SELECT departments.dept_name, employees.name
FROM departments
LEFT JOIN employees ON departments.dept_id = employees.dept_id;

3. Auditing and Data Review

Administrators may use LEFT JOIN to identify inconsistencies in data, such as missing relational tags or category mappings.

Performance Considerations

From a performance perspective, there is no difference between using LEFT JOIN and LEFT OUTER JOIN because the SQL engine processes them the same way. Behind the scenes, the query planners in most database systems parse both expressions to the same semantic execution plan.

The only thing that may influence performance are:

• Indexes on join keys
• Table size and data distribution
• Join filtering conditions

So while JOIN syntax makes no difference in performance, query structure and indexing strategies do.

Best Practices and Recommendations

Although you can use either LEFT JOIN or LEFT OUTER JOIN, here are a few best practices for consistency and clarity:

• Be consistent: Choose one style and stick to it throughout your codebase for readability.
• Use OUTER for teaching: When explaining JOINs to newcomers, some prefer using the full syntax (LEFT OUTER JOIN) to draw contrast with other JOIN types.
• Write for your audience: In team environments or corporate settings, follow internal guidelines or conventions.

Conclusion

At its core, the distinction between LEFT JOIN and LEFT OUTER JOIN is semantic, not functional. They refer to the same operation in SQL, and choosing one over the other is often a matter of style or convention, not requirement. Understanding this subtlety helps not only reduce confusion but also contributes to writing clearer, more maintainable SQL queries.

Whether you’re analyzing user behavior, generating reports, or ensuring data integrity, LEFT JOINs offer a flexible and powerful way to match and preserve data from separate sources. Now that you know there’s no operational difference, you’re free to choose the expression that best fits your use case or aligns with your team’s coding standards.