Explicit vs. Implicit SQL Joins: Do They Differ in Performance?

Explicit vs. Implicit SQL Joins: A Performance Analysis

SQL joins come in two primary forms: explicit and implicit. Implicit joins, also known as "old-style joins," utilize a comma (,) within the FROM clause, omitting the explicit JOIN keyword. Conversely, explicit joins leverage the JOIN syntax, offering enhanced readability and more precise control.

A common question arises: do these join types differ significantly in performance? The short answer is generally no.

Let's examine illustrative queries:

Explicit Join Example:

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

Implicit Join Example:

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

In databases like SQL Server, these queries yield identical results and exhibit comparable execution times. Therefore, performance differences are negligible.

It's crucial to remember that implicit outer joins (using =* or =* in the WHERE clause) are outdated and discouraged since SQL Server 2005. However, implicit (cross) joins, as demonstrated above, remain supported.

In conclusion, performance shouldn't dictate the choice between explicit and implicit joins. Readability, maintainability, and developer preference are typically the deciding factors.

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