Correlated subqueries

Correlated subqueries in SQL are nested queries where the inner query references a column from the outer query. These subqueries are evaluated for each row processed by the outer query.

Consider a table named Employees with columns EmployeeID, EmployeeName, and Salary. We want to retrieve all employees whose salary is greater than the average salary of their department.

SELECT EmployeeID, EmployeeName, Salary FROM Employees e1 WHERE Salary > ( SELECT AVG(Salary) FROM Employees e2 WHERE e1.DepartmentID = e2.DepartmentID );

In this example, the subquery (SELECT AVG(Salary) FROM Employees e2 WHERE e1.DepartmentID = e2.DepartmentID) is correlated because it references the DepartmentID column from the outer query (e1). For each row processed by the outer query, the subquery calculates the average salary of the corresponding department. The outer query then retrieves the EmployeeID, EmployeeName, and Salary where the salary is greater than the average salary of the department. This query returns employees whose salary exceeds the average salary of their department.

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