Outer Joins | Learn SQL Visual

Keep Rows Without a Match

An outer join keeps rows that have no matching partner. LEFT JOIN keeps every left-side row and fills unmatched right-side columns with NULL. FULL JOIN keeps unmatched rows from both sides.

In a LEFT JOIN, what happens to right-side columns when no right row matches?

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Columns used for the join

categories5 rows

id	name
1	Electronics
2	Phones
3	Laptops
4	Books
5	Fiction

products5 rows

name	category_id
Pixel Phone	2
iPhone	2
ThinkPad	3
MacBook	3
SQL Guide	5

Compare categories.id on the left with products.category_id on the right. An outer join can keep rows that have no matching partner.