Explicit vs. Implicit SQL Inner Joins: Is There a Performance Difference?

Explicit vs. Implicit SQL Inner Joins: A Performance Deep Dive

SQL offers two ways to combine data from multiple tables: explicit and implicit joins. Both achieve the same result, but do they differ in performance? Let's investigate.

Explicit Joins: Clarity and Precision

Explicit joins use the INNER JOIN keyword to clearly define the join condition:

<code class="language-sql">SELECT * 
FROM table_a INNER JOIN table_b
ON table_a.id = table_b.id;</code>

This approach enhances readability and maintainability, making the join logic immediately apparent.

Implicit Joins: The Concise Alternative

Implicit joins leverage the WHERE clause to implicitly specify the join condition:

<code class="language-sql">SELECT table_a.*, table_b.*
FROM table_a, table_b
WHERE table_a.id = table_b.id;</code>

While more compact, this style can be less clear, especially in complex queries.

Performance: A Tale of Two Approaches

In practice, the performance difference between explicit and implicit inner joins is negligible, at least within the context of SQL Server. Both methods generally execute with comparable efficiency.

Important Note: Deprecated Syntax

It's vital to remember that implicit OUTER JOIN syntax using =* or =* in the WHERE clause is outdated and unsupported in SQL Server 2005 and later. However, the comma-separated implicit (CROSS) JOIN syntax, as illustrated above, remains valid. Choosing explicit joins is generally recommended for improved code clarity and future compatibility.

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