Write SQL Like a Pro: 9 Tips for Clean and Readable Code

You’ve mastered the basics of SQL and can retrieve the data you need. That’s a great start! But writing good SQL isn’t just about getting the result; it’s also about making your code easy to understand, maintain, and debug. Clean SQL queries are a hallmark of a skilled data professional.

Why bother with clean SQL? Imagine trying to decipher a complex query written by someone else (or even your past self!) without any formatting or clear structure. It can be a nightmare! Clean queries save time, reduce errors, and make collaboration much smoother. Let’s explore 9 practical tips to help you write cleaner SQL.

Enhancing Readability

1. Use Meaningful Table & Column Aliases

Aliases act like nicknames for your tables and columns, making your queries shorter and more intuitive.

• Why? They improve readability, especially when dealing with long table or column names, or when joining tables with the same column names.
• Example:

  Before:

  SELECT customers.customer_name, orders.order_date
  FROM customers
  JOIN orders ON customers.customer_id = orders.customer_id;
  

  After:

  SELECT c.customer_name, o.order_date
  FROM customers AS c
  JOIN orders AS o ON c.customer_id = o.customer_id;
  

  • Here, c is an alias for customers, and o is an alias for orders. It’s much cleaner!

2. Always Indent Your SQL Clauses

Proper indentation visually structures your query, making it easier to follow the flow of logic.

• Why? It clearly separates different parts of your SQL statement, such as SELECT, FROM, WHERE, GROUP BY, HAVING, and ORDER BY.
• Example:

  Before:

  SELECT customer_name, COUNT(order_id) FROM customers JOIN orders ON customers.customer_id = orders.customer_id WHERE order_date >= '2024-01-01' GROUP BY customer_name ORDER BY COUNT(order_id) DESC;
  

  After:

  SELECT
      c.customer_name,
      COUNT(o.order_id)
  FROM
      customers AS c
  JOIN
      orders AS o ON c.customer_id = o.customer_id
  WHERE
      o.order_date >= '2024-01-01'
  GROUP BY
      c.customer_name
  ORDER BY
      COUNT(o.order_id) DESC;
  

  • The indented structure makes it immediately clear what each part of the query does.

3. Use UPPERCASE for SQL Keywords

Using uppercase for SQL keywords (like SELECT, FROM, WHERE, JOIN, etc.) helps them stand out from table and column names, improving readability.

• Why? It creates a visual distinction between the language’s built-in commands and the user-defined elements.
• Consistency is key: While not strictly mandatory in most SQL dialects, maintaining a consistent case for keywords makes your code look more professional and easier to parse.

Writing Efficient and Focused Queries

4. Avoid SELECT *

While SELECT * (selecting all columns) might seem convenient, it’s generally considered bad practice for several reasons.

• Why avoid it?
  • Performance: It retrieves more data than you might actually need, potentially slowing down your query.
  • Readability: It’s not immediately clear which columns are being used in subsequent parts of the query.
  • Maintenance: If the table structure changes (columns are added or removed), your query might break or return unexpected results.
• Best Practice: Explicitly list the columns you need in your SELECT statement.

5. Use JOIN Only When Needed

Joins are powerful for combining data, but unnecessary joins can lead to performance issues and make your queries harder to understand.

• Why be selective? Only join tables that are essential for retrieving the data you need for the specific query.
• Think before you join: Ask yourself if you truly need columns from another table to fulfill the query’s purpose.
• Example: If you only need customer names, there’s no need to join the orders table.

Structuring Complex Queries

6. Format Long Queries for Readability

Long and complex SQL queries can become very difficult to read if they’re just one continuous line of code.

• Strategies:
  • Break down long WHERE clauses: Use line breaks and indentation for multiple conditions.
  • Align JOIN conditions: Align the ON clauses with the respective JOIN statements.
  • Separate subqueries: Format subqueries clearly with their own indentation.

7. Leverage CTEs for Complex Logic

As mentioned in our previous post on learning SQL effectively [Internal Link to the previous blog post], Common Table Expressions (CTEs) are excellent for breaking down complex logic into smaller, more manageable, and named steps.

• Why use CTEs?
  • Improved Readability: They make complex queries easier to understand by separating different logical blocks.
  • Reusability: You can reference a CTE multiple times within the same query.
  • Maintainability: Changes to a specific part of the logic are isolated within the CTE.

  Example:

  WITH MonthlySales AS (
      SELECT
          STRFTIME('%Y-%m', order_date) AS sale_month,
          SUM(order_amount) AS total_sales
      FROM
          orders
      GROUP BY
          sale_month
  )
  SELECT
      sale_month,
      total_sales
  FROM
      MonthlySales
  ORDER BY
      sale_month;
  

  • The MonthlySales CTE clearly defines the logic for calculating monthly sales before the final SELECT statement retrieves the results.

Adding Clarity and Efficiency

8. Add Comments to Tricky Parts

Don’t hesitate to add comments to your SQL queries, especially for sections that might be confusing or require explanation.

• Why add comments?
  • Explanation: Clarify the purpose of specific clauses or complex logic.
  • Context: Provide context for why certain decisions were made in the query.
  • Future Reference: Help yourself and others understand the code later.
• Types of comments: Most SQL dialects support single-line comments (using --) and multi-line comments (/* ... */).

9. Filter Early with WHERE

Applying filters using the WHERE clause as early as possible in your query can significantly improve performance.

• Why filter early? By reducing the number of rows that need to be processed in subsequent operations (like joins and aggregations), you can make your queries run faster.
• Think about the data flow: Filter out unnecessary data before joining or grouping it.

Conclusion: Writing SQL with Intention

Writing clean SQL queries is a skill that develops over time with practice and attention to detail. By adopting these 9 tips, you’ll not only write more readable and maintainable code but also become a more efficient and effective SQL user. Remember that good SQL is about clear communication – making it easy for anyone (including your future self) to understand and work with your queries.

Ready to elevate your SQL writing skills?

• Explore SQL style guides and best practices from reputable sources like SQL Style Guide by Kenneth Downs (External Link).
• Practice rewriting your existing SQL queries using these tips.
• Share your own tips for writing clean SQL in the comments below! What are your go-to strategies for readable code?