SQL Joins: Comma Notation vs. ANSI JOIN Syntax – Which Approach Should You Use?

Understanding the Differences in SQL Join Table Approaches

In SQL, joining tables enables you to retrieve related data from multiple tables. However, there are two common approaches to join tables: using the comma notation (also known as the "OLD JOIN" syntax) and using the ANSI JOIN syntax.

OLD JOIN Syntax vs. ANSI JOIN Syntax

The main difference between the two approaches lies in their syntax:

-- Comma Notation
select user.name, post.title 
  from users as user, posts as post 
 where post.user_id = user.user_id;

-- ANSI JOIN Syntax
select user.name, post.title 
  from users as user join posts as post
 using user_id;

For small datasets, both approaches yield the same results. However, the ANSI JOIN syntax is recommended as the standard syntax that aligns with industry best practices.

Semantic Differences

Semantically, the comma notation produces a Cartesian product between the tables, resulting in all possible combinations of rows. The WHERE clause is then used to filter out the desired rows.

In contrast, the JOIN syntax explicitly defines the link between the tables through the ON clause, making the join operation more explicit.

Functional Differences

Functionally, the two approaches perform similarly. However, using the comma notation may lead to performance issues or unexpected results when using other JOIN types (e.g., LEFT JOIN, RIGHT JOIN). These issues are avoided by using the ANSI JOIN syntax instead.

MySQL-Specific Difference

MySQL specifically allows the use of the STRAIGHT_JOIN keyword with the comma notation, which forces MySQL to read the left table before the right table. However, it is generally not advisable to rely on this behavior for performance optimization.

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