SQL Joins: What are the Performance and Syntax Differences Between USING, ON, and WHERE?

SQL JOIN methods: performance and syntax differences of USING, ON and WHERE

SQL provides several options for joining tables: USING, ON and WHERE. Although the functionality of these options is similar, misunderstandings about their performance and syntactic meaning may arise. This article explores the differences between these connection methods.

Performance

Contrary to popular belief, there are no significant performance differences between the three connection methods. All three methods produce the same optimized query plan.

Syntax and semantics

ANSI syntax:

• JOIN ​​(introduced in ANSI-92) is the recommended syntax for performing joins. It uses the ON clause to explicitly define the join conditions.
• WHERE (ANSI-89) allows the use of a WHERE clause to specify join conditions, but it has been deprecated due to potential readability and ambiguity issues.

USING syntax:

• USING (introduced in SQL:2003) is a shorthand syntax that uses the USING keyword to specify common columns between joined tables. It provides concise and intuitive syntax.

Ambiguity of “join the where” (implicit INNER JOIN)

Using a WHERE clause to specify join conditions can lead to ambiguity, especially in outer join types. The problem stems from the logical query processing order in SQL Server, where FROM is evaluated before WHERE. "join the where" may inadvertently create a Cartesian product or produce unexpected results.

Grammatical ambiguity:

The following two queries illustrate the syntax ambiguities that can arise when mixing implicit inner joins in the WHERE clause with explicit outer joins in the FROM clause:

FROM T1, T2, T3
WHERE T1.ID = T2.ID AND T1.foo = 'bar' AND T2.fish = 42

FROM T1 
INNER JOIN T2 ON T1.ID = T2.ID
INNER JOIN T3 ON T1.ID = T3.ID
WHERE T1.foo = 'bar' AND T2.fish = 42

The ambiguity arises from the inability to clearly determine which connections are external and which are internal.

Recommendation:

For clarity, consistency, and compatibility with modern SQL standards, it is recommended:

• Use explicit joins (JOIN syntax with ON clause) for all joins.
• Avoid using "join the where" (implicit INNER JOIN in WHERE clause).
• Use ANSI-92 syntax (JOIN with ON clause) for cross-platform compatibility.

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