CROSS JOIN vs. Comma-Separated Joins in SQL: What's the Difference?

SQL Table Joins: Comma-Separated vs. CROSS JOIN

Combining tables in SQL often involves using either a comma between table names or the CROSS JOIN operator. While seemingly interchangeable, key differences exist.

Syntax Comparison

Comma-Separated Join:

SELECT * FROM tableA, tableB;

CROSS JOIN:

SELECT * FROM tableA CROSS JOIN tableB;

Both create a Cartesian product—every row in tableA is paired with every row in tableB.

Semantic Equivalence and Syntactic Differences

Functionally, both methods are semantically equivalent, producing the same Cartesian product. However, the comma-separated join is outdated legacy syntax. CROSS JOIN, conforming to the SQL-92 standard, is the preferred and more readable approach, especially when working with OUTER JOINs, which are not possible with comma-separated joins.

Performance and Best Practices

While inherent performance differences are negligible, CROSS JOIN is recommended for better compatibility with modern database systems and adherence to current SQL standards. This improves code readability and reduces potential future compatibility issues.

Summary

Both comma-separated joins and CROSS JOINs yield identical results—a Cartesian product. However, CROSS JOIN is the superior choice because of its SQL-92 compliance and broader functionality. Remember that Cartesian products can lead to unexpectedly large result sets if not carefully considered. Always understand the implications of creating a Cartesian product before using either method.

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