Common Table Expressions (CTEs)

An In-Depth Guide

Common Table Expressions, or CTEs, are a powerful SQL feature that allow you to build complex queries in a modular and readable way. In this article, we’ll dive deep into CTEs—covering everything from basic usage to advanced recursive queries—and provide a variety of examples to help you leverage their full potential.

What Are CTEs?
Basic Syntax and Simple Examples
Chaining Multiple CTEs
Recursive CTEs for Hierarchical Data
Advanced Use Cases and Examples
Practical Tips and Best Practices
Conclusion

What Are CTEs?

A Common Table Expression (CTE) is a temporary, named result set that you can reference within a SQL statement. They were introduced in the SQL:1999 standard and provide a cleaner, more organized alternative to subqueries, especially when dealing with complex logic or recursive data relationships.

Key Advantages:

Improved Readability: Break down intricate queries into manageable parts.
Reusability: Reference the same temporary result multiple times within a query.
Recursive Capabilities: Easily navigate hierarchical data, such as organizational charts or folder structures.
Simplified Maintenance: Modular code is easier to modify and debug.

Basic Syntax and Simple Examples

The Basic Structure

A CTE is defined using the WITH keyword, followed by a name and a query that produces the result set. The CTE is then available for the main query that follows.

WITH cte_name AS (
    -- Define the CTE query here
    SELECT column1, column2, ...
    FROM table_name
    WHERE condition
)
SELECT *
FROM cte_name;

Example 1: Filtering Data

Assume you have an employees table, and you want to retrieve information about employees earning over 50,000.

WITH high_earners AS (
    SELECT employee_id, first_name, last_name, salary
    FROM employees
    WHERE salary > 50000
)
SELECT *
FROM high_earners;

In this example, the CTE named high_earners isolates the employees that meet the salary condition, and the final SELECT retrieves all columns from this temporary result set.

Chaining Multiple CTEs

CTEs can be defined in a chain, allowing you to build up complex data transformations in stages.

Example 2: Multi-Step Data Transformation

Imagine you have a sales table and want to first filter recent sales and then calculate the total sales per region.

WITH recent_sales AS (
    SELECT sale_id, region, amount, sale_date
    FROM sales
    WHERE sale_date >= '2025-01-01'
),
regional_totals AS (
    SELECT region, SUM(amount) AS total_sales
    FROM recent_sales
    GROUP BY region
)
SELECT *
FROM regional_totals
ORDER BY total_sales DESC;

This query first creates a CTE called recent_sales to filter the data, then uses it in the regional_totals CTE to aggregate sales by region, and finally displays the results ordered by total sales.

Recursive CTEs for Hierarchical Data

Recursive CTEs enable you to work with hierarchical data where a row in a table refers to another row in the same table.

Structure of a Recursive CTE

A recursive CTE consists of:

Anchor Member: The initial query that seeds the recursion.
Recursive Member: A query that references the CTE itself to build upon the result set.

Example 3: Employee Hierarchy

Suppose you have an employees table with employee_id, manager_id, and employee_name columns. To list all employees under a specific manager, you might write:

WITH RECURSIVE employee_hierarchy AS (
    -- Anchor member: starting with the top-level manager
    SELECT employee_id, employee_name, manager_id
    FROM employees
    WHERE employee_id = 1  -- Top-level manager's ID

    UNION ALL

    -- Recursive member: select employees reporting to those already found
    SELECT e.employee_id, e.employee_name, e.manager_id
    FROM employees e
    INNER JOIN employee_hierarchy eh ON e.manager_id = eh.employee_id
)
SELECT *
FROM employee_hierarchy;

This recursive CTE begins with a specific manager and repeatedly joins the table to find all direct and indirect reports.

Advanced Use Cases and Examples

Example 4: Data Aggregation and Transformation

CTEs are useful when performing complex calculations such as running totals or moving averages. Consider a scenario where you need to compute a running total of transactions.

WITH ordered_transactions AS (
    SELECT 
        transaction_id,
        amount,
        transaction_date,
        ROW_NUMBER() OVER (ORDER BY transaction_date) AS rn
    FROM transactions
),
running_totals AS (
    SELECT 
        t1.transaction_id,
        t1.amount,
        t1.transaction_date,
        SUM(t2.amount) AS running_total
    FROM ordered_transactions t1
    JOIN ordered_transactions t2 ON t2.rn <= t1.rn
    GROUP BY t1.transaction_id, t1.amount, t1.transaction_date
)
SELECT *
FROM running_totals
ORDER BY transaction_date;

This example illustrates how to use a CTE to assign a row number to transactions and then calculate a cumulative sum of the amounts up to each transaction.

Example 5: Using CTEs with DML Statements

CTEs are not limited to SELECT queries—they can also be applied in INSERT, UPDATE, and DELETE operations. For example, suppose you want to update the salaries of employees who have received an "Excellent" performance rating, incorporating a 10% bonus.

WITH bonus_calculation AS (
    SELECT 
        employee_id,
        salary,
        salary * 0.10 AS bonus
    FROM employees
    WHERE performance_rating = 'Excellent'
)
UPDATE employees
SET salary = salary + bonus
FROM bonus_calculation
WHERE employees.employee_id = bonus_calculation.employee_id;

In this query, the bonus_calculation CTE computes the bonus for each eligible employee, and the main UPDATE statement applies these bonuses to update the salary.

Practical Tips and Best Practices

Meaningful Names: Give your CTEs descriptive names that clearly convey their purpose.
Modularity: Break down large queries into several smaller CTEs for easier debugging and maintenance.
Performance Considerations: While CTEs improve clarity, they can sometimes have performance implications. Compare execution plans and consider indexing or query refactoring if needed.
Recursion Limits: Be aware that many SQL engines set a default recursion limit (often 100 iterations). Ensure your recursive queries have proper termination conditions to prevent infinite loops.
Temporary Scope: Remember that CTEs exist only for the duration of the query in which they are defined—they are not permanent database objects.

Conclusion

CTEs offer an elegant way to manage complex SQL queries by decomposing them into more manageable parts. Whether you’re filtering data, performing multi-step aggregations, or working with hierarchical structures using recursive queries, mastering CTEs can significantly enhance both the readability and maintainability of your SQL code.

By understanding the nuances of CTEs and exploring various practical examples, you can take full advantage of this versatile feature in your daily database operations. Happy querying!

Last modified: 03 March 2025

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