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Advanced SQL: A Guide to Subqueries and Common Table Expressions (CTEs)

Advanced SQL: A Guide to Subqueries and Common Table Expressions (CTEs)

While basic SQL commands like `SELECT` and `WHERE` are essential, the real power of SQL for complex data analysis lies in its ability to nest queries. **Subqueries** and **Common Table Expressions (CTEs)** are two powerful tools that allow you to perform multi-step calculations and simplify complex logic within a single SQL statement. Understanding when and how to use them is a key step toward becoming a proficient data analyst or backend developer. 📊

Subqueries: Queries within Queries

A **subquery** (also known as a nested query) is a `SELECT` statement that is embedded within another SQL statement. The inner query is executed first, and its result is then used by the outer query. Subqueries are often used in the `WHERE` clause to filter data based on the results of another query.

Example: Finding all employees whose salary is above the company's average salary.

SELECT first_name, last_name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

The inner query `(SELECT AVG(salary) FROM employees)` first calculates the average salary, and the outer query then uses that value to filter the employees.

CTEs: The Cleaner Alternative

A **Common Table Expression (CTE)** is a temporary, named result set that you can reference within a `SELECT`, `INSERT`, `UPDATE`, or `DELETE` statement. CTEs are often preferred over subqueries for their readability and reusability, especially when a query involves multiple steps or needs to be referenced more than once.

Example: Using a CTE to find the same employees as the previous example.

WITH AverageSalary AS (
    SELECT AVG(salary) AS avg_sal
    FROM employees
)
SELECT first_name, last_name, salary
FROM employees, AverageSalary
WHERE salary > avg_sal;

The `WITH` keyword defines the CTE, giving it a clear name (`AverageSalary`). This makes the code much easier to read and debug. While both subqueries and CTEs can accomplish similar tasks, CTEs are generally the go-to for complex queries due to their superior organization and readability.

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