Are Comma-Style Joins in SQL Less Efficient Than Explicit JOIN Keywords?

Comparison of the efficiency of comma joins and explicit JOIN keywords in SQL

Early SQL queries, when developers were not familiar with the JOIN keyword, often used the following format:

SELECT a.someRow, b.someRow
FROM tableA AS a, tableB AS b
WHERE a.ID = b.ID AND b.ID = $someVar

As developers start using INNER JOIN, a question arises: Do these old queries need to be rewritten?

Performance impact

While this query is syntactically valid, it can impact performance in some cases. Consider the following query:

SELECT * FROM people p, companies c
WHERE p.companyID = c.id AND p.firstName = 'Daniel'

Many databases will first calculate the Cartesian product of all rows in the two tables before filtering. For large data sets this would be very inefficient.

Best Practices

To avoid these performance issues, it is recommended to use the JOIN keyword explicitly. The following rewritten query organizes the constraints more efficiently:

SELECT * FROM people p JOIN companies c ON p.companyID = c.id
WHERE p.firstName = 'Daniel'

This method directly instructs the database to perform join operations instead of calculating a complete Cartesian product, thereby increasing speed and reducing memory footprint.

Summary

While old queries using the "comma join" syntax will work fine, it is recommended to update them to use the JOIN keyword. Doing so not only improves readability but also resolves potential performance issues.

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