How Do Comma Notation and ANSI-JOIN Syntax Differ in SQL Queries?

What's the Difference Between the Comma Notation and ANSI-JOIN Syntax in SQL Queries?

Consider two tables, Users and Posts, where user_id is the foreign key in Posts and the primary key in Users. The following SQL queries join these tables to retrieve the user's name and post's title:

-- 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;

Syntax Differences

The primary difference between these queries lies in their syntax. The comma notation, an older approach, uses a comma to separate table names, while the ANSI-JOIN syntax employs the JOIN keyword followed by ON or USING to specify the join condition.

Semantic Differences

Comma Notation:

• Semantically, the comma notation represents a Cartesian product.
• This means it creates a matrix of all records from one table with all records from the other, resulting in a large intermediate result set before applying the WHERE clause.

ANSI-JOIN Syntax:

• The ANSI-JOIN syntax explicitly defines the join condition, clearly indicating how the tables are linked.
• It follows the ANSI standard, which is considered more precise and consistent.

Practical Differences

For these specific queries, the two approaches yield the same results. However, when using other JOIN types (e.g., OUTER JOINs), the ANSI-JOIN syntax is preferred due to its clear and unambiguous syntax.

Additional Note for MySQL

In MySQL, the comma notation has a slight difference with the STRAIGHT_JOIN keyword. However, it's not advisable to rely on this minor distinction for join operations.

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