Left join and left outer join in SQL: Differences

SQL can be confusing sometimes, especially when names seem to say the same thing. One question people often ask is: What’s the difference between LEFT JOIN and LEFT OUTER JOIN? It’s a good question. Let’s break it down in simple terms and have some fun doing it!

Contents

TL;DR: Quick Summary

LEFT JOIN and LEFT OUTER JOIN are exactly the same in SQL. There’s no difference in behavior or result. The word “OUTER” is optional and mostly used for clarity. So, don’t worry — whichever you use, you’re doing it right!

What is a JOIN in SQL?

Let’s start from the basics. A JOIN is used in SQL to combine rows from two or more tables. It helps you pull data that relates to each other based on common columns — usually keys.

Imagine you have two tables:

  • Customers – contains customer info like ID and name.
  • Orders – contains order info including which customer placed which order.

If you want to see every customer and the orders they placed, you need a JOIN!

Types of JOINs

There are several types of JOINs in SQL:

  • INNER JOIN – Only gets rows that match in both tables.
  • LEFT JOIN – Gets all rows from the left table and matching ones from the right.
  • RIGHT JOIN – The opposite of LEFT JOIN.
  • FULL OUTER JOIN – Gets all rows from both tables, even if there’s no match.

Today, we’re focusing on LEFT JOIN and LEFT OUTER JOIN.

LEFT JOIN vs LEFT OUTER JOIN

Okay, you’ve seen both written down — LEFT JOIN and LEFT OUTER JOIN. You might think they do different things. But here’s the magic:

They are exactly the same.

Yes, really. SQL lets you omit the word “OUTER” when doing a LEFT JOIN. Adding it just makes the intention clearer, especially in complex queries where other JOINs are involved.

Both queries below do the same thing:

SELECT * 
FROM Customers 
LEFT JOIN Orders ON Customers.ID = Orders.CustomerID;

SELECT * 
FROM Customers 
LEFT OUTER JOIN Orders ON Customers.ID = Orders.CustomerID;

Same logic. Same output.

What Does a LEFT JOIN Actually Do?

With a LEFT JOIN, you get:

  • All rows from the table on the left.
  • Matching rows from the table on the right.
  • If there’s no match on the right, SQL fills the blanks with NULL.

This makes LEFT JOIN super useful when you want everything from Table A, and optional data from Table B.

Example Time!

Take a look at this sample data:

Customers:

ID Name
1 Alice
2 Bob
3 Charlie

Orders:

OrderID CustomerID Amount
101 1 $50
102 1 $30
103 2 $70

If we run a LEFT JOIN on Customers and Orders, we’ll get Alice, Bob, and Charlie. Even though Charlie hasn’t placed any orders!

This is great for reports. You can see who hasn’t ordered anything yet.

Why Does OUTER Even Exist?

The keyword OUTER is mostly there for completeness and readability. SQL also has:

  • FULL OUTER JOIN
  • RIGHT OUTER JOIN

If you use OUTER in those, it makes sense to also write LEFT OUTER JOIN. It keeps everything consistent. But again — it’s optional!

Think of it as using “please” in a sentence. Friendly and polite — but not always necessary.

When Should You Use LEFT JOIN?

Here are some times when it’s really handy:

  • Find customers with or without orders.
  • Get products and see if they were sold.
  • List events and check for attendees, even if no one showed up.

Anywhere you want to include all rows from the left side, whether or not there’s data on the right side.

Gotchas To Watch Out For

LEFT JOIN is powerful, but here are a few things to remember:

  • NULLs – You may see a lot of NULL values in columns from the right table. That’s not wrong, just means there’s no match.
  • WHERE clause – Be careful using WHERE clauses with LEFT JOINs. They can accidentally turn your LEFT JOIN into an INNER JOIN if you filter on the right table’s columns.

Example of a Mistake:

SELECT *
FROM Customers
LEFT JOIN Orders ON Customers.ID = Orders.CustomerID
WHERE Orders.Amount > 50;

This query will only show people who have orders — so Charlie is out. LEFT JOIN wasted!

To fix it, move the filter into the JOIN or use conditions that allow NULLs.

SELECT *
FROM Customers
LEFT JOIN Orders 
  ON Customers.ID = Orders.CustomerID 
  AND Orders.Amount > 50;

Now Charlie is still in the game — with NULLs if he didn’t order anything!

Other Aliases and Shortcuts

Some databases and systems allow more relaxed syntax. For example:

  • LEFT OUTER JOIN can become just LEFT JOIN.
  • Most modern SQL tools and developers actually default to LEFT JOIN.

So don’t panic if you see them mixed and matched. Pick your style!

Final Thoughts

So here’s the takeaway:

  • LEFT JOIN = LEFT OUTER JOIN. No difference!
  • Use LEFT JOIN to include all rows from the left.
  • “OUTER” is just for clarity, not function.

Knowing your JOINs makes you a SQL ninja. Keep practicing and try different combinations. Your data has stories to tell — JOINs help you hear them.

Now go out there and LEFT JOIN with confidence!

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