Does MySQL Default to INNER JOIN or Comma Syntax?

MySQL Default Join Behavior: Unraveling the Mystery

In the realm of MySQL, the absence of an explicit JOIN clause raises a fundamental question: what is the default behavior? Is it akin to an INNER JOIN, or does it emulate the older comma-separated syntax?

The Default: An INNER JOIN

Contrary to popular belief, leaving out the JOIN clause in MySQL is effectively the same as employing an INNER JOIN. In other words, the specification of INNER is redundant. MySQL interprets such statements as INNER JOINs, enabling you to omit it for brevity.

Comparing JOIN Variations

To clarify further, consider the following scenarios:

• SELECT * FROM t1 JOIN t2 is equivalent to SELECT * FROM t1 INNER JOIN t2.
• SELECT * FROM t1, t2 emulates the comma-separated syntax, similar to SELECT * FROM t1 INNER JOIN t2.

WHERE Clauses and Joins

Regarding your second inquiry, WHERE clauses have a nuanced relationship with JOINs. While they share similarities, they serve distinct purposes.

WHERE clauses filter the data retrieved from the participating tables. In contrast, JOINs define the relationships between those tables, determining which rows are included in the result set.

Comma Syntax vs. JOIN Syntax

Although the effect of using commas and keywords (JOIN or INNER JOIN) is identical, their history and implications differ. The comma syntax, inherited from the ANSI-89 standard, has some drawbacks that prompted the introduction of the JOIN keyword in the ANSI-92 standard.

Benefits of Using JOIN Syntax

For clarity and maintainability, it is highly advisable to consistently use JOIN syntax over commas. This offers several advantages:

• Improved readability by separating table relationships and filters.
• Enhanced maintainability by keeping these elements distinct.
• Seamless conversion to OUTER JOINs.
• Avoidance of ambiguous errors due to precedence rules.
• Reduced likelihood of creating cartesian products by omitting join clauses.

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