Explicit vs. Implicit SQL Joins: What's the Performance Difference?

Explicit vs. Implicit SQL Joins: A Performance Comparison

Understanding Explicit and Implicit SQL Joins

SQL queries often involve joining multiple tables. This can be achieved using either explicit or implicit join syntax. Both methods link tables based on common columns, but their stylistic differences have sometimes led to questions about performance.

Explicit joins use the INNER JOIN keyword to clearly define the join condition. Implicit joins, conversely, use a comma (,) to separate table names, potentially making the query less readable.

Here's a comparison:

Explicit Join:

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

Implicit Join:

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

Performance Implications

In SQL Server (and many other modern database systems), there's no substantial performance difference between explicit and implicit INNER joins. The database optimizer handles both in a similar manner, resulting in the same logical execution plan.

Important Notes

It's crucial to remember that implicit OUTER join syntax (using *= or =* in a WHERE clause with a comma-separated table list) is outdated and deprecated in SQL Server 2005 and later. However, implicit (CROSS) joins using commas, as shown in the example above, remain supported.

Choosing the Right Approach

The decision between explicit and implicit joins often boils down to coding style and readability. Explicit joins enhance clarity and maintainability, while implicit joins might appear more compact. The optimal choice depends on individual developer preferences and project coding standards.

The above is the detailed content of Explicit vs. Implicit SQL Joins: What's the Performance Difference?. For more information, please follow other related articles on the PHP Chinese website!