CTE (Common Table Expression) is a temporary result set defined within the execution scope of a single SELECT, INSERT, UPDATE, or DELETE statement.

It’s similar to a subquery, but:

• It improves readability and modularity
• Supports recursive operations
• Makes complex queries easier to debug

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Basic Syntax of a CTE：

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

You can also chain multiple CTEs:

WITH cte1 AS (...),
     cte2 AS (...)
SELECT ...
FROM cte2;

Why Business & Data Analysts Should Use CTEs

Advantage	Benefit for Analysts
✅ Readability	Break down complex logic into named steps
✅ Reusability	Avoid repeating the same logic multiple times
✅ Debuggability	Easier to test parts of queries
✅ Recursion	Solve hierarchical data problems (e.g. org charts)
✅ Compatibility	Works in PostgreSQL, SQL Server, Snowflake, BigQuery

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Real-World Use Cases for CTEs

Let’s walk through business-relevant CTE examples, each with SQL code and analyst-focused explanations.

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Use Case 1: Top-N Customers by Revenue

Problem: Find the top 5 customers by revenue in the last year.

WITH customer_sales AS (
    SELECT
        customer_id,
        SUM(order_total) AS total_revenue
    FROM orders
    WHERE order_date >= DATEADD(year, -1, GETDATE())
    GROUP BY customer_id
)
SELECT *
FROM customer_sales
ORDER BY total_revenue DESC
LIMIT 5;

Why CTE helps: You separate aggregation logic from the final filter/order, making the query easier to manage.

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Use Case 2: Filtered Aggregations

Problem: Compare sales from new vs returning customers.

WITH first_orders AS (
    SELECT customer_id, MIN(order_date) AS first_order
    FROM orders
    GROUP BY customer_id
),
classified_orders AS (
    SELECT o.order_id, o.customer_id, o.order_total,
           CASE WHEN o.order_date = f.first_order THEN 'New' ELSE 'Returning' END AS customer_type
    FROM orders o
    JOIN first_orders f ON o.customer_id = f.customer_id
)
SELECT customer_type, SUM(order_total) AS total_sales
FROM classified_orders
GROUP BY customer_type;

Why CTE helps: You clearly separate logic into order classification and summary aggregation.

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Use Case 3: Recursive CTE for Hierarchies

Problem: Retrieve an entire management chain from an employee table.

WITH RECURSIVE org_chart AS (
    SELECT employee_id, manager_id, name, 1 AS level
    FROM employees
    WHERE employee_id = 101  -- starting employee

    UNION ALL

    SELECT e.employee_id, e.manager_id, e.name, oc.level + 1
    FROM employees e
    JOIN org_chart oc ON e.employee_id = oc.manager_id
)
SELECT * FROM org_chart;

Why CTE helps: Recursive CTEs are ideal for organizational structures, product category trees, and dependency chains.

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Use Case 4: Breaking Down Funnel Steps

Problem: Analyze conversion rates in a marketing funnel.

WITH pageviews AS (
    SELECT user_id FROM events WHERE event_name = 'PageView'
),
signups AS (
    SELECT user_id FROM events WHERE event_name = 'Signup'
),
purchases AS (
    SELECT user_id FROM events WHERE event_name = 'Purchase'
)
SELECT
    (SELECT COUNT(*) FROM pageviews) AS total_views,
    (SELECT COUNT(*) FROM signups) AS total_signups,
    (SELECT COUNT(*) FROM purchases) AS total_purchases;

Why CTE helps: Separate each funnel step as a clean building block, great for dashboards and presentations.

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Use Case 5: Simplify Repetitive Joins

Problem: Join the same derived table to multiple other tables.

WITH sales_summary AS (
    SELECT product_id, SUM(sales_amount) AS total_sales
    FROM sales
    GROUP BY product_id
)
SELECT p.product_name, s.total_sales
FROM products p
JOIN sales_summary s ON p.product_id = s.product_id;

Why CTE helps: You avoid repeating subqueries for common aggregations or filters.

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CTE vs Subquery: What’s the Difference?

Feature	CTE	Subquery
Reusability	✅ Named, reusable	❌ One-time use only
Readability	✅ Better for complex logic	😐 Harder to read/debug
Nesting	❌ Can’t be nested easily	✅ Can nest deeply
Recursion	✅ Supported	❌ Not supported
Compatibility	✅ Widely supported	✅ Universal

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Best Practices When Using CTEs

• Keep each CTE focused on one task
• Avoid creating too many levels (no more than 3–4 chained CTEs)
• Comment each CTE with what it does
• Use CTEs for clarity, not for performance optimization (unless recursive)
• Use indexes on base tables for large CTEs to improve performance

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✅ Summary

Concept	Why It Matters for Analysts
CTE Basics	Create temporary named query results
Modularity	Write complex logic in readable blocks
Real Use Cases	Customer revenue, funnel analysis, org charts
Recursive CTE	Solve hierarchical problems elegantly
Best Practices	Keep it clean, reusable, and documented