Comma-Separated vs. CROSS JOIN: Which SQL Syntax Should You Use for Cartesian Products?

Understanding the Differences Between Comma-Separated and CROSS JOIN SQL Syntax for Cartesian Products

Database operations often require combining data from multiple tables. This article examines two methods for creating Cartesian products: comma-separated syntax and the CROSS JOIN operator.

Comma-Separated Joins

The statement SELECT * FROM A, B uses commas to join tables A and B, generating a Cartesian product. This means each row in table A is paired with every row in table B, irrespective of any relationships between them.

CROSS JOIN Syntax

The statement SELECT * FROM A CROSS JOIN B explicitly uses the CROSS JOIN operator to achieve the same Cartesian product result as the comma-separated method.

Performance Comparison

Interestingly, there's no substantial performance difference between these two approaches. Both produce all possible row combinations from the involved tables.

Choosing the Right Syntax

Despite their functional equivalence, CROSS JOIN is generally preferred. This is because it adheres to the SQL-92 standard, unlike the comma-separated method (which is considered SQL-89 syntax). The older comma-separated syntax lacked native support for outer joins (LEFT, RIGHT, FULL), leading to database-specific implementations and portability issues. SQL-92 standardized outer join syntax, making CROSS JOIN the more consistent and portable choice.

Summary

While both syntaxes create identical Cartesian products, CROSS JOIN is the recommended approach for its SQL-92 compliance, ensuring consistent behavior across various database systems and better compatibility with outer join operations.

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