Common Table Expressions (CTEs) are a powerful feature in SQL that improve the readability and structure of complex queries, making data analysis easier to manage and interpret. CTEs are particularly useful for breaking down multi-step data transformations into simpler, modular pieces, offering a cleaner alternative to traditional subqueries.

What is a CTE?

A CTE is a temporary result set that you define within a WITH clause and can be referenced by the main query that follows. Unlike traditional subqueries, CTEs are often more readable because they allow you to assign a name to the result set, making complex queries easier to understand. The basic syntax is:

WITH cte_name AS (
    SELECT ...
)
SELECT ...
FROM cte_name;

Benefits of Using CTEs

Readability: CTEs allow you to structure queries into logical sections, making them more readable and easier to debug. For example, instead of nesting multiple subqueries within the FROM clause, you can define each step in a separate CTE, making it clear what each part of the query is doing.
Performance: CTEs are often more efficient than inline subqueries because they are executed once and stored in memory. This means that if you reference the CTE multiple times in your query, it does not need to be recalculated each time
Modularity: When analyzing data with complex calculations—like calculating moving averages, cumulative sums, or complex joins—CTEs can break down the problem into manageable steps, each with a clear purpose. This modularity makes it easier to edit or expand the analysis over time.

Example: Using CTEs for Data Analysis

Let’s say you want to analyze sales data and calculate the monthly revenue along with a cumulative total. Using a CTE simplifies the process:

WITH monthly_sales AS (
    SELECT 
        DATE_TRUNC('month', sale_date) AS sale_month,
        SUM(amount) AS total_sales
    FROM sales
    GROUP BY sale_month
)
SELECT 
    sale_month,
    total_sales,
    SUM(total_sales) OVER (ORDER BY sale_month) AS cumulative_sales
FROM monthly_sales;

In this example, the monthly_sales CTE calculates the sales per month, making it easier to use this result set in the final query to compute the cumulative sales. This reduces code repetition and enhances the clarity of each calculation step.

Refactoring Subqueries with CTEs

As you work with SQL, subqueries can become long and complex, making it difficult to track each part of the query. CTEs offer a way to manage this complexity by allowing you to declare the subquery ahead of the main query using the WITH clause. Instead of wrapping subqueries inside a FROM statement, a CTE is named and referenced like any other table.

For example, a query that initially uses a subquery in the FROM clause to filter matches with 10 or more goals could be rewritten using a CTE:

WITH high_scoring_matches AS (
    SELECT 
        m.id, m.country_id
    FROM matches AS m
    WHERE (m.home_goal + m.away_goal) >= 10
)
SELECT 
    c.name, 
    COUNT(hsm.id) AS high_scoring_matches_count
FROM high_scoring_matches AS hsm
JOIN country AS c ON hsm.country_id = c.id
GROUP BY c.name;

In this refactored query, the CTE high_scoring_matches isolates the logic of filtering matches with high goal counts, making the main query simpler and more focused.

When to Use CTEs

Recursive Queries: CTEs support recursion, making them a great tool for querying hierarchical data like organizational charts or tree structures.
Breaking Down Complex Queries: When dealing with multiple layers of filtering, aggregation, and transformation, CTEs help to organize these steps clearly.
Data Cleaning and Transformation: CTEs can help preprocess data (like filtering out invalid entries) before performing further analysis.

Final Thoughts

CTEs are an excellent tool for data analysts and database developers, offering a clean, reusable, and understandable way to manage complex SQL queries. They provide a balance between simplicity and functionality, making SQL-based data analysis more efficient.

Using CTEs for SQL Data Analysis