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Exploring Subquery Alternatives: Understanding and Using CTE
- Subqueries are queries nested within another SQL query, often used to perform intermediate calculations or data filtering.
- Common Table Expressions (CTEs) are temporary named result sets defined within a SQL statement using the WITH keyword.
- Subqueries are hard to read, challenging to maintain, and sometimes frustratingly slow. Debugging deeply nested queries, in particular, was a headache.
- CTEs simplify complex queries by breaking them into smaller, reusable components, improving readability and maintainability.
- In this article, the author shared a case study to highlight the differences between subqueries and CTEs.
- The case study involves managing financial data related to sellers, specifically their transactions and withdrawals.
- The article shows a subquery approach and a CTE approach to solve the problem of calculating the running balance for the selected seller_id.
- The subquery approach calculates the running balance using a correlated subquery, which is executed repeatedly for each row in the outer query leading to significant performance overhead.
- The CTE-based approach efficiently calculates the cumulative balance_after using a window function that processes the dataset in a more optimized manner compared to a correlated subquery.
- A recommendation is made that the CTE-based approach should be used in scenarios where performance is critical, especially with larger datasets.
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