Independent Subquery and
Correlated Subquery
In Independent subquery ,Inner query is independent of outer query.
This article will present you with a complete idea about Independent sub query and Correlated sub query.
We will learn below topics in this article
Independent Subquery
Inner query is executed first and the results are stored then Outer query then runs on the stored results.
If a subquery does use any refrences from outer query then it would be called as Independent Subquery.
it is also called Incorrelated sub query.
Example:
Consider Employee table having below records.
Find minimum salary of the Employee in a company
SELECT Employee_FirstName, Employee_LastName,Salary
FROM Employee
WHERE salary = (SELECT MIN (salary) FROM Employee)
Correlated Subquery
- Inner query depends on outer query
- Outer query needs to be executed before the inner query.
- The inner query is executed separately for each row of the outer query.
- These queries are slower compare to independent sun query
- Data from each row in the main query is passed to the subquery for processing
Syntax
SELECT ColumnNames from Table1Â t1 WHERE column1Â
[Comparison Operator]
(SELECT ColumnNames
FROM Table2 WHERE exp1 = t1.exp2); Example:
Consider Employee table having below records.
Find all the employees who have more than average salary in their department.
SELECT Employee_FirstName,Employee_LastName, Salary, Department_id
FROM Employee e1
WHERE salary > (SELECT AVG(salary)
FROM Employee
WHERE DepartmentID = e1.DepartmentID);
