difference between subquery and correlated subquery in oracle - Wikitechy

What is the difference between the SubQuery and Corelated SubQuery ?

Editor — Sun, 18 Jul 2021 20:30:02 +0000

Subquery

The inner query is executed only once.
The inner query will get executed first and the output of the inner query used by the outer query.
The inner query is not dependent on outer query.

SELECT Employee_name, dept_no 
FROM Employee 
WHERE Employee_name IN (SELECT Employee_name FROM Customer);

Correlated SubQuery

The outer query will get executed first and for every row of outer query, inner query will get executed.
So the inner query will get executed as many times as number of rows in result of the outer query.
The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other.

SELECT Employee_name,dept_id 
FROM Employee
WHERE Employee_name IN (SELECT Employee_name FROM dept WHERE Employee.dept_id=dept.dept_id);