SQL Joins: Comma-Delimited or Explicit JOIN Syntax – Which is Better?

SQL Joins: Comma-Delimited Notation vs. Explicit JOIN Syntax

In the realm of SQL database querying, we often encounter scenarios where we need to combine data from multiple tables. Two common approaches to achieve this are comma-delimited notation (also known as implicit joins) and explicit JOIN syntax. Let's explore their differences.

Consider the following scenario: we have two tables, "Users" and "Posts," with "user_id" as the foreign key in the "Posts" table and primary key in the "Users" table. To retrieve the user's name and post title, we can use the following queries:

1. Comma-Delimited Notation

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

2. Explicit JOIN Syntax

select user.name, post.title
from users as user join posts as post
using user_id;

Semantic Differences

Syntactically, the comma notation appears more compact and intuitive. However, semantically, it differs from explicit JOIN syntax. The comma notation produces a Cartesian product, creating a large temporary table that contains all possible combinations of rows from the two tables, potentially resulting in excessive data duplication. Even though MySQL optimizes the execution to avoid the Cartesian product, the semantics remain different.

On the other hand, the explicit JOIN syntax clearly defines the join condition with the ON clause, explicitly specifying how the rows from the two tables are related. This is particularly crucial when using outer joins, as comma notation does not support them.

Functional Equivalence and Other Considerations

Apart from the semantic differences, both approaches will produce identical results for the given queries. However, it is generally recommended to favor explicit JOIN syntax because it is more explicit, easier to read, and portable across different databases. Moreover, it enables the use of different join types (e.g., LEFT JOIN, RIGHT JOIN), providing greater flexibility in data retrieval.

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