Are Comma-Separated Joins in SQL/MySQL Queries Less Efficient Than Keyword Joins?

In SQL/MySQL queries, is comma-separated join less efficient than keyword join?

When writing database queries, you often encounter syntax that uses commas to separate joins without using the JOIN keyword. Although this method looks similar to INNER JOIN, it may affect efficiency. This article explores the reasons for using the JOIN keyword and its impact on query performance.

Comma connection and keyword connection

Traditionally, joins in SQL/MySQL can be achieved by listing the tables using commas (,) and then filtering using the WHERE clause. This method is called "comma concatenation":

<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>

However, the introduction of the JOIN keyword provides a specialized syntax to explicitly define joins:

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

Performance Difference

While comma concatenation technically accomplishes the same task as keyword concatenation, performance differences may occur under specific circumstances. In particular, using a WHERE clause in a comma join for unconstrained filtering can lead to inefficiencies.

For example, consider the following comma concatenation:

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

Most databases will interpret this query as first performing the Cartesian product of the people and companies tables. Then, apply the WHERE clause to identify matching records. This approach involves significant overhead since the Cartesian product can result in huge data sets.

Advantages of keyword linking

Using the JOIN keyword has the following advantages:

• Explicit join specification: The JOIN keyword clearly indicates the type of join being performed, improving code readability and maintainability.
• Performance improvements: Keyword joins, especially joins with explicit ON clauses, allow the database to optimize query execution by directly calculating the required join results without generating unnecessary intermediates data.
• Modern syntax: The JOIN keyword is the standard syntax for defining joins in SQL and MySQL, ensuring compatibility with current and future versions of database systems.

Suggestion

Based on performance considerations and the advantages of keyword joins, it is recommended to use the JOIN keyword in all queries. While existing comma join queries may not require immediate modification, keyword syntax is recommended to comply with modern standards and improve code quality.

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