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

Jointing Tables: Examining Two SQL Query Approaches

In database management, joining tables is crucial for combining data from multiple tables. This article explores the nuances between two common joining table approaches, using SQL queries to illustrate the differences.

Query 1: Using Comma Notation

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

This query employs comma notation to perform a Cartesian product between the Users and Posts tables. It then filters the resulting rows based on the equality of post.user_id and user.user_id.

Query 2: Using ANSI-JOIN Syntax

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

In this query, the ANSI-JOIN syntax is employed, which provides a clearer definition of how the tables interact. The ON clause explicitly specifies the joining condition, in this case, user_id.

Semantic Differences

Syntactic differences aside, these queries yield identical results in simple scenarios. However, the underlying semantics differ. Comma notation implies a Cartesian product, while the ANSI-JOIN approach emphasizes specific table relationships.

Functional Differences

For these particular queries, the functionality is essentially the same. The distinction becomes apparent when utilizing other JOIN types, such as OUTER JOINs.

MySQL-Specific Difference

In MySQL specifically, comma-notation offers a subtle difference: it allows for the use of STRAIGHT_JOIN, which ensures that the left table is always read before the right table. While this can be beneficial in certain scenarios, it is generally not a recommended approach.

Recommendation

When possible, it is preferable to use ANSI-JOIN syntax as it is the ANSI standard and provides greater clarity in defining table relationships. Comma notation remains an option, but it is important to be mindful of its semantic implications and potential nuances in specific database systems.

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