Cross Join vs. Comma-Separated Tables: What's the Difference and Which Should I Use?

Understanding Cross Joins and Comma-Separated Table Joins in SQL

SQL offers two methods for combining tables: cross joins and comma-separated table listings. While seemingly interchangeable, significant differences exist.

Cross Join vs. Comma Separation: A Comparison

The core difference lies in their syntax:

• Cross Join: SELECT * FROM A CROSS JOIN B
• Comma-Separated: SELECT * FROM A, B

Output Analysis

Without a WHERE clause, both produce an identical result: a Cartesian product—every row from table A paired with every row from table B.

Syntax Standards and Modern Practices

The comma-separated approach is considered outdated (SQL-89 standard), superseded by the CROSS JOIN operator introduced in SQL-92. Although many databases accept both, CROSS JOIN is the standard in modern SQL.

Performance Considerations

Generally, performance discrepancies between these methods are negligible.

The Advantages of the SQL-92 Standard

The SQL-92 syntax offers a crucial benefit:

• OUTER JOIN Compatibility: SQL-92 introduced OUTER JOINs (LEFT JOIN, FULL JOIN, RIGHT JOIN), enabling precise row inclusion control from each table. The older comma-separated syntax lacks this capability.

Conclusion

While both techniques generate Cartesian products, CROSS JOIN is recommended for its clarity, adherence to modern SQL standards, and essential support for OUTER JOIN operations. Using CROSS JOIN ensures better code readability and maintainability.

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