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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.
SELECT empnum, name FROM employee as e1 WHERE salary > (SELECT avg(salary) FROM employee WHERE department = e1.department);
In the above query the outer query is,
SELECT empnum, name FROM employee as e1 WHERE salary >
And the inner query is,
(SELECT avg(salary) FROM employee WHERE department = e1.department);
In the above nested query the inner query has to be executed for every employee as the department will change for every row. Hence the average salary will also change.
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