ANSI JOIN vs. Non-ANSI JOIN in T-SQL: What are the Performance Differences?

Performance difference between ANSI JOIN and non-ANSI JOIN in T-SQL

In T-SQL stored procedures, non-ANSI JOIN syntax has always been common:

SELECT A.A, B.B, C.C
FROM aaa AS A, bbb AS B, ccc AS C
WHERE
    A.B = B.ID
AND B.C = C.ID
AND C.ID = @param

There is speculation that replacing this with ANSI-92 JOIN syntax could improve performance:

SELECT A.A, B.B, C.C
FROM aaa AS A
JOIN bbb AS B
   ON A.B = B.ID
JOIN ccc AS C
   ON B.C = C.ID
   AND C.ID = @param

Are they the same?

In this particular example, the execution of both queries is identical. However, ANSI-92 syntax has several advantages:

• Clarity: The JOIN clause separates logic into relations and filters, enhancing readability.
• Avoid ambiguity: In some cases, non-ANSI outer joins can lead to ambiguous results or deadlocks. ANSI-92 syntax eliminates this situation.
• Standards Compliance: ANSI-92 is the current industry standard and has been adopted by most developers and query tools.
• Avoiding unexpected Cartesian products: The JOIN clause helps prevent unexpected Cartesian products.

Arguments with non-ANSI syntax

Some argue that non-ANSI syntax helps conceptualize SQL as a Cartesian product followed by filtering. While this technique can aid understanding, the preferred ANSI-92 syntax is recommended because it is generally clearer and standards-compliant. The only exception to this rule is when using natural joins, which should be avoided entirely due to their inherent risks.

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