What is a CROSS JOIN, and what result does it produce?

Difficulty: 5/10

joins, Cartesian product, performance

Understanding CROSS JOIN in MySQL

A CROSS JOIN in MySQL returns the Cartesian product of two tables. This means every row from the first table is combined with every row from the second table.

1. What CROSS JOIN Does

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• Produces all possible combinations of rows between two tables.
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• No ON condition is used (though MySQL allows it syntactically).
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• If table A has X rows and table B has Y rows, the result has X × Y rows.

2. Example of CROSS JOIN

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Example:
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SELECT *
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FROM colors
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CROSS JOIN sizes;
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3. When to Use CROSS JOIN

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• To generate combinations (e.g., product variations).
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• To create test datasets.
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• When explicitly needed for Cartesian product logic.

In short, a CROSS JOIN creates every possible row combination between two tables, which can grow very large quickly if the tables contain many rows.

Scenario Questions

0-2 years experience

Follow-up Questions