Mastering SQL: 9 Best Practices for Writing Efficient Queries

Optimizing SQL queries is crucial for improving performance and readability in any database-driven application. In this blog, we will walk through key SQL query best practices, offering actionable tips to help developers write cleaner, faster, and more maintainable code.

SQL (Structured Query Language) is the backbone of relational databases, enabling us to interact with data seamlessly. However, writing effective SQL queries isn't just about getting the right results; it's also about optimizing performance, ensuring scalability, and keeping the code readable for future developers.

In this post, we’ll cover essential techniques that will improve your SQL query writing, making your code more efficient and easier to manage.

1. Use SQL Keywords in Capital Letters

When writing SQL queries, it's a good practice to capitalize all SQL keywords such as SELECT, WHERE, JOIN, etc. This simple step improves readability, making it easier for other developers to quickly understand the structure and logic of your query.

Example:

2. Use Table Aliases with Columns When Joining Multiple Tables

Aliases provide a shorthand for table names, making your queries easier to read, especially when dealing with long or multiple table names. Aliases also help prevent ambiguity when columns from different tables share the same name.

Example:

In this example, e and d are aliases that make the query shorter and cleaner.

3. Never Use SELECT *; Always Specify Columns in SELECT Clause

Using SELECT * might seem convenient, but it fetches all columns, which can be inefficient, especially in tables with many columns. Instead, explicitly specify the columns you need to reduce unnecessary data retrieval and enhance performance.

Example:

By only selecting the necessary columns, you optimize both performance and clarity.

4. Add Useful Comments for Complex Logic, But Avoid Over-Commenting

While it's important to comment on complex logic to explain your thought process, over-commenting can clutter the query. Focus on adding comments only when necessary, such as when the logic might not be immediately clear to others.

Example:

5. Use Joins Instead of Subqueries for Better Performance

Joins are often more efficient than subqueries because they avoid the need to process multiple queries. Subqueries can slow down performance, especially with large datasets.

Example:

6. Create CTEs Instead of Multiple Subqueries for Better Readability

Common Table Expressions (CTEs) make your queries easier to read and debug, especially when working with complex queries. CTEs provide a temporary result set that you can reference in subsequent queries, improving both readability and maintainability.

Example:

7. Use JOIN Keywords Instead of Writing Join Conditions in WHERE Clause

Using the JOIN keyword makes your SQL queries more readable and semantically clear, rather than placing join conditions in the WHERE clause.

Example:

8. Never Use ORDER BY in Subqueries, It Increases Runtime Unnecessarily

Using ORDER BY in subqueries can cause unnecessary performance hits since the sorting is often unnecessary at that point. Instead, apply ORDER BY only when absolutely needed in the final result set.

Example:

9. Use UNION ALL Instead of UNION When You Know There Are No Duplicates

The UNION operator removes duplicates by default, which adds overhead to your query. If you're certain there are no duplicates, using UNION ALL can greatly improve performance by skipping the duplicate-checking step.

Example:

By following these best practices, you’ll be able to write queries that run faster, are easier to understand, and scale well as your database grows. Small optimizations like avoiding SELECT *, using proper joins, and leveraging CTEs can make a big difference in the long run. 

Happy querying❗

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