Скачать или смотреть How to Refactor SQL Queries: Remove EXCEPT and Use WHERE Clauses Instead

Discover how to efficiently refactor your SQL queries by eliminating `EXCEPT` and using `WHERE` conditions in this comprehensive guide. Perfect for SQL Server users looking to enhance their query performance.
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How to Refactor SQL Queries: Remove EXCEPT and Use WHERE Clauses Instead

SQL queries can sometimes become complex and difficult to read, especially when they rely on operations like EXCEPT. If you're working with SQL Server and looking to streamline your query by eliminating EXCEPT, you're in the right place. This post will break down the original SQL query and provide a cleaner alternative using WHERE clauses.

The Problem

You may find yourself with a SQL query that utilizes the EXCEPT operator when you would like to simplify it. The following SQL chunk is an example of such a scenario:

[[See Video to Reveal this Text or Code Snippet]]

In the above SQL statement, the goal is to update records in the PriceValues table, but the EXCEPT operation makes the query more complicated than necessary.

The Solution

To refactor the query and remove EXCEPT, you can use a combination of joins and the NOT EXISTS clause. Here’s how to transform the original query.

Step-by-Step Refactoring

Use a Normal Join: Instead of using EXCEPT, you'll directly join the relevant tables.

Implement the NOT EXISTS Clause: This gives you the equivalent condition you need without the complexity of EXCEPT.

Here’s the refactored query:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes in the Query

Joining with -RowsIWant: This replaces the EXCEPT operation by directly joining with the -RowsIWant temporary table.

Condition Replaced by NOT EXISTS: The original conditions to filter out certain records are now handled using NOT EXISTS, which checks if there are no matching records fulfilling the specified criteria.

Consider Other Options

LEFT JOIN with IS NULL: Another option to achieve similar results could be using LEFT JOIN with IS NULL. However, this approach might not be as performant as using NOT EXISTS, and could lead to increased complexity in the query.

Avoid NOT IN: It’s crucial to avoid using NOT IN, especially when dealing with potential null values, as it can lead to unexpected results.

Conclusion

Refactoring SQL queries to remove operators like EXCEPT can greatly enhance readability and performance. By leveraging JOIN conditions and NOT EXISTS, you can keep your queries clean and efficient. This not only simplifies the code but also ensures that your intentions are clear to anyone reviewing your work or when you revisit your code later.

By following this guide, you can improve not only your queries but your overall SQL proficiency. Happy querying!