SQL Left Joins

A LEFT OUTER JOIN (or simply LEFT JOIN) returns all records from the left table, along with matching records from the right table. If no match is found, columns from the right table are filled with NULL. This join ensures that all rows from the left table are preserved, even if no corresponding records exist in the right table.

In other words, grab everything from Table A, as well as anything in Table B that is also in Table A.

SELECT * FROM TableA
LEFT OUTER JOIN TableB
ON TableA.col_match = TableB.col_match

Note that we can also use the left join to find rows that are unique to Table A using a WHERE clause:

SELECT * FROM TableA
LEFT OUTER JOIN TableB
ON TableA.col_match = TableB.col_match
WHERE TableB IS null

Example

Given two tables:

Customers

id	name	city
1	Alice	New York
2	Bob	Chicago
3	Charlie	San Diego
4	David	Miami

Orders

id	customer_id	product
1	1	Laptop
2	1	Keyboard
3	3	Monitor
4	5	Mouse

Executing a LEFT JOIN:

SELECT Customers.name, Orders.product  
FROM Customers  
LEFT JOIN Orders ON Customers.id = Orders.customer_id;

Result:

name	product
Alice	Laptop
Alice	Keyboard
Charlie	Monitor
Bob	NULL
David	NULL

Explanation:

• Alice and Charlie had matching orders, so they appear with their respective products.
• Bob and David had no matching orders, so NULL appears in the product column.
• The order for customer ID 5 (Mouse) is not included, since it has no match in Customers.

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Key Differences from Other Joins

Join Type	Includes Unmatched Rows?	Where Do NULLs Appear?
INNER JOIN	❌ No	No unmatched rows
LEFT JOIN	✅ Yes (left table)	Right table columns
RIGHT JOIN	✅ Yes (right table)	Left table columns
FULL OUTER JOIN	✅ Yes (both tables)	Both sides where no match exists

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Related Notes

• SQL Inner Joins
• SQL Right Joins
• SQL Full Outer Joins