Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A correlated sub-query is a term used for specific types of queries in SQL in computer databases. 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.

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