What is MySQL\'s Default JOIN Behavior: INNER or OUTER?

MySQL's Default JOIN Behavior: INNER or OUTER?

Question:

When using the JOIN keyword in a MySQL query without specifying an explicit type, what is the default behavior: INNER JOIN or OUTER JOIN?

Answer:

INNER JOIN is the default behavior in MySQL. This means that when you write:

SELECT * FROM t1 JOIN t2

It is equivalent to:

SELECT * FROM t1 INNER JOIN t2

Related Question: JOIN vs. WHERE

You also inquired about the relationship between JOIN and WHERE clauses. A WHERE clause filters the results of a query based on a specified condition, while a JOIN operation combines rows from multiple tables based on a specified join condition.

While the effect of a stand-alone JOIN may appear similar to using commas and WHERE clauses, there are subtle differences to consider:

• Comma syntax is part of the ANSI-89 standard, while JOIN syntax was introduced in the ANSI-92 standard to address limitations with commas.
• JOIN syntax provides greater readability, maintainability, and extensibility, especially when working with OUTER JOINs.
• Mixing comma and JOIN syntax in the same statement can result in unexpected errors.

Recommendation:

It is highly recommended to always use JOIN syntax instead of commas for clarity, maintainability, and error prevention.

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