Understanding the Efficiency Difference Between Explicit and Implicit SQL Joins

When working with SQL, one of the common tasks you might encounter is joining tables. However, a question often arises among developers and data analysts: Is there any efficiency difference between explicit and implicit joins in SQL? This blog post aims to clarify this question and provide a comprehensive understanding of the two join methods.

What Are SQL Joins?

SQL joins are a way to combine records from two or more tables in a database based on related columns. There are several types of joins, the most common being:

• Inner Join: Combines rows from two tables that match on a specified condition.
• Outer Join: Includes not only the matched rows but also the unmatched ones from one table.

In this post, we will focus specifically on the inner join, which can be expressed in two primary ways: explicit and implicit.

Explicit Inner Join

An explicit inner join uses the INNER JOIN clause to join tables. Here’s an example:

SELECT * FROM
table a 
INNER JOIN table b ON a.id = b.id;

In this example, we clearly state that we are joining table a and table b on the condition that a.id equals b.id.

Implicit Inner Join

An implicit inner join uses a comma to list tables and a WHERE clause for the join condition. Here’s how it looks:

SELECT a.*, b.*
FROM table a, table b
WHERE a.id = b.id;

This method uses a comma to indicate that we’re combining two tables, followed by a condition in the WHERE clause.

Performance Comparison

Now, let’s get to the crux of the matter: performance. The initial question was whether there’s any efficiency difference between these two methods in SQL Server. The answer is no—performance-wise, they are exactly the same. This means that whether you opt for an explicit inner join or an implicit inner join, SQL Server processes both in the same way.

Important Considerations

While both methods perform equally, there are some points to keep in mind:

• Readability: Many developers favor explicit joins for their clarity. The explicit syntax makes it immediately clear which tables are being joined and under what conditions.
• Deprecated Syntax: It’s important to note that while the implicit syntax using commas is still supported, the use of implicit outer joins—which utilize *= or = * in the WHERE clause—has been deprecated since SQL Server 2005. Thus, using explicit joins is more future-proof and reduces the risk of errors in your queries.

References

For further information, you can read about the Deprecation of “Old Style” JOIN Syntax.

Conclusion

In conclusion, while both explicit and implicit inner joins are efficient in SQL Server, using explicit joins can contribute to better code readability and maintainability. Understanding these nuances helps in writing clearer and more effective SQL queries.

Whether you’re a new developer or a seasoned data analyst, being aware of SQL syntax and best practices will help you leverage the full power of databases effectively.