MySQL Joins: What\'s the Default, and Why Should I Care?

MySQL Default Join Behavior: Unraveling the Mystery

As a programmer navigating the MySQL realm, it's crucial to understand the default join behavior to ensure efficient and accurate data retrieval. This article explores the differences between unqualified JOIN, comma (,) syntax, and WHERE clauses in MySQL, providing clear answers to your pressing questions.

What is the Default MySQL Join Behavior?

Unlike many other SQL dialects, MySQL defaults to INNER JOIN when using an unqualified JOIN statement. This means only matching rows between two tables are returned, where the join condition is satisfied.

Therefore, the following two queries are equivalent:

<code class="sql">SELECT * FROM t1 JOIN t2;
SELECT * FROM t1 INNER JOIN t2;</code>

What About Comma Syntax?

The comma syntax (,), also inherited from the ANSI-89 standard, serves a similar purpose to JOIN. However, it suffers from several drawbacks, including:

• Less readability compared to JOIN syntax
• Difficulty in converting to OUTER JOIN statements
• Potential for incorrect cartesian products due to missing join clauses

Conclusion

While comma syntax has historical significance, it's strongly recommended to always use JOIN syntax for clarity, maintainability, and consistency. MySQL's default INNER JOIN behavior can be easily modified using OUTER JOIN clauses when necessary. By embracing JOIN syntax, you ensure precise data retrieval and avoid common pitfalls associated with comma syntax.

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