What is the order of execution process of subquery in SQL?

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Khushi Singh · Answer

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 andgt; (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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What is the order of execution process of subquery in SQL?

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