What is the difference between a subquery and a CTE in SQL?
Sep 16, 2025 am 07:47 AM
Subqueries are queries nested in another query. They are suitable for simple one-time calculations and can be located in SELECT, FROM or WHERE clauses; 2. CTE is defined through WITH clauses to improve the readability of complex queries and supports recursion and multiple references; 3. Subqueries are suitable for single use, and CTE is more suitable for scenarios where clear structure, reuse or recursion is required.
A subquery and a CTE (Common Table Expression) both allow you to write modular, reusable SQL logic within a larger query, but they differ in structure, readability, and use cases.
Subquery
A subquery is a query nested inside another query. It can appear in the SELECT , FROM , or WHERE clause. Subqueries are enclosed in parentses and executed first when possible, with results passed to the outer query.
Key characteristics:
- Can be used inline and are often best for simple, one-off computings.
- May be correlated (dependent on the outer query) or non-correlated.
- Harder to read and maintain when deeply nested.
- Limited reusability—must be repeated if needed multiple times.
SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);
CTE (Common Table Expression)
A CTE is a named temporary result set defined using the WITH clause. It's not stored but can be referenced multiple times in the main query.
Key characteristics:
- Improves readability by breaking complex queries into clear parts.
- Supports recursion (eg, traversing hierarchical data).
- Can be referenced multiple times without rewriting.
- Exists only during the execution of the query.
WITH avg_salary AS ( SELECT AVG(salary) AS avg_sal FROM employees ) SELECT name FROM employees, avg_salary WHERE salary > avg_sal;
Basically, use a subquery for simple, single-use logic and a CTE when you need clarity, reuse, or recursion. Both serve similar purposes but CTEs are generally better for complex or multi-step queries.
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