Why Are JOIN Keywords Preferred Over Comma Joins in SQL?

Why Avoid Comma Joins in SQL?

New SQL developers often use comma joins, a less explicit way to join tables:

<code class="language-sql">SELECT a.someRow, b.someRow 
FROM tableA AS a, tableB AS b 
WHERE a.ID=b.ID AND b.ID= $someVar</code>

While functional (equivalent to an INNER JOIN), this method has potential performance drawbacks.

The Inefficiency of Comma Joins

Consider this example:

<code class="language-sql">SELECT * FROM people p, companies c 
    WHERE p.companyID = c.id AND p.firstName = 'Daniel'</code>

The database might initially create a full Cartesian product (every combination of people and companies rows), then filter based on the WHERE clause. This is significantly less efficient than alternative methods.

The Superiority of Keyword-Based Joins

Using explicit JOIN keywords and the ON clause offers a more efficient and readable approach:

<code class="language-sql">SELECT * FROM people p JOIN companies c ON p.companyID = c.id
    WHERE p.firstName = 'Daniel'</code>

This clearly indicates the join condition to the database, optimizing the join process and avoiding the unnecessary creation of a large intermediate Cartesian product. This leads to faster query execution and reduced resource consumption, especially with large datasets.

Best Practice: Always Use JOIN Keywords

For better performance, readability, and maintainability, always prefer JOIN keywords (INNER JOIN, LEFT JOIN, RIGHT JOIN, etc.) over comma joins. While existing comma joins might not immediately need rewriting, adopting this best practice consistently improves your SQL code.

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