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What is correlated sub-query?

Posted By :volvo     Posted Date :November 28, 2013    Points :40   Category :Sql Server 
A correlated sub-query is a term used for specific types of queries in SQL. It is a sub-query (a query nested inside another query) that uses values from the outer query in its WHERE clause. The sub-query is evaluated once for each row processed by the outer query.

Here is an example for a typical correlated sub-query. In this example we are finding the list of employees (employee number and names) having more salary than the average salary of all employees in that employee's department.

SELECT employee_number, name
FROM employee AS e1
WHERE salary > (SELECT avg(salary)
FROM employee
WHERE department = e1.department);

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