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Home Forums What is the order of execution process of subquery in SQL? – MindStick
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1 Answers

Khushi Singh
Khushi Singh 07 Jan 2025
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Order of Execution Process for Subqueries in SQL

Non-Correlated Subqueries:

The subquery runs before producing a result set to be utilized by the outer query.
Example: Subquery located in the WHERE or HAVING clauses.

SELECT employee_id 
FROM employees
WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'IT');

Correlated Subqueries:

The outer query executes first, and the subquery runs for each row of the outer query.

Example:

SELECT e1.employee_id 
FROM employees e1
WHERE salary > (SELECT AVG(salary) FROM employees e2 WHERE e1.department_id = e2.department_id);

Subqueries in FROM Clause (Inline Views):

The subquery executes as a temporary table, then the outer query processes its result.

SELECT department_id, MAX(avg_salary)
FROM (SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id) sub;

Subqueries in SELECT Clause:

The outer query executes, and the subquery runs for each row.

SELECT employee_id, (SELECT department_name FROM departments WHERE department_id = employees.department_id) AS department_name FROM employees;

Summary:


Non-Correlated: Subquery runs first.

Correlated: Outer query begins first and then comes the subquery and it works row wise.

FROM Clause: Subquery is treated as a temporary table that is formally known as derived table.

SELECT Clause: Subquery occurs for each record of the outer query on the database.


 

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