Comma Joins vs. Explicit JOINs in SQL: Are Comma Joins a Performance Bottleneck?

SQL comma join and explicit JOIN: performance bottleneck analysis

When writing database queries, it is easy to ignore the explicit JOIN keyword and use the more familiar comma-separated syntax:

<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>

As you get deeper into SQL basics, you may come across explicit JOIN syntax and wonder if the comma-separated queries you were using earlier caused any problems.

Risk of comma concatenation

Although the comma join syntax is functionally equivalent to INNER JOIN, in some common cases it can cause unforeseen performance issues. Consider the following example:

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

In this query, the database will calculate the Cartesian product of the people and companies tables before applying the filter. This unconstrained Cartesian product only exists in memory and is short-lived, but its calculation can be very time-consuming.

Better approach: explicit JOIN

A more efficient approach is to use the JOIN keyword to group constraints with a JOIN:

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

This approach allows the database to directly compute fully constrained JOINs, avoiding unnecessary Cartesian product calculations.

Summary

While there is nothing inherently wrong with comma joins, it is generally recommended to use explicit JOIN syntax. Not only does this make the code easier to read and maintain, but it can also improve performance in some cases. By using the JOIN keyword, you can optimize your SQL queries and ensure they run as efficiently as possible.

The above is the detailed content of Comma Joins vs. Explicit JOINs in SQL: Are Comma Joins a Performance Bottleneck?. For more information, please follow other related articles on the PHP Chinese website!