Correlated vs Non-Correlated Subqueries
In SQL database management systems, database table-based generated subqueries can be of a correlated or non-correlated nature. It is very important for database users to understand the main difference between correlated and non-correlated subqueries, as this directly impacts the subqueries created by the database user, whether the user-created subquery runs or not, and how the correlated or non-correlated output subquery is directly or indirectly related to the outer query.
So, let’s understand correlated and non-correlated subqueries better in SQL database management systems.
Non-Correlated Subqueries in SQL Databases.
A non-correlated subquery generated in an SQL database table is a subquery that can be executed or run independently in the outer query. In a non-correlated subquery, the database table subquery does not reference any column of the outer query, and the output result is analyzed and performed once in the detailed outer query.
Elements of Non-Correlated Subqueries.
- Execution – A non-correlated subquery is executed or run only once, and the output result of the non-correlated subquery is used in the outer query.
- Independence – A non-correlated subquery is independent in nature, and it does not completely depend on any outer query for its result.
- Performance – non-correlated subqueries are more efficient and effective because they are executed only once.
Example of a Non-Correlated Subquery in a Database Table.
Here, assume the database user has two database tables named Employee and Department.
- This is the Employee table, which has the table column fields employee_id, emp_name, and salary.
- This is the Department table, which has the table column fields department_id and department_name.
Here, the database user wants to extract the names of employees who earn more than the average salary of all employees in the Employee table.
SELECT emp_name
FROM employee
WHERE salary > (
SELECT AVG(salary)
FROM employee
);
Explanation of Non-Correlated Subqueries.
- Here, the subquery SELECT AVG(salary) FROM employee calculates the average salary for all employees in the Employee table.
- The outer query then compares each employee’s salary with the calculated average salary.
- A non-correlated subquery has independent behaviour, and it executes only once in the database statement to display the average salary.
Correlated Subqueries in SQL Databases.
In SQL database management systems, a database user-generated correlated subquery directly references or indicates one or more columns of the outer query. This means the correlated subquery is completely dependent on the data values of the outer query, and the database table executes once for each row of the outer query.
Elements of Correlated Subqueries.
- Execution – Remember that a correlated subquery created in a database table executes multiple times during execution, it runs once for each table row in the correlated outer query.
- Dependency – The correlated subquery is completely dependent on the outer query, specifically, on the values of the current row of the outer table query.
- Performance – Correlated subqueries process or run slower compared to non-correlated subqueries because correlated subqueries need to be executed multiple times.
Example of a Correlated Subquery.
Here, let’s assume the database user wants to find a list of employees from the employee table who earn more than the average salary of their department. The user has tables named employee and department as output.
SELECT emp_name
FROM employee e
WHERE salary > (
SELECT AVG(salary)
FROM employee
WHERE department_id = e.department_id
);
Explanation of Correlated Subquery.
- Here, the correlated subquery (SELECT AVG(salary) FROM employee WHERE department_id = e.department_id) calculates the average salary for each employee’s department.
- The outer query extracts the names of employees from the table whose salary is greater than the average salary of their respective department.
- The subquery is correlated because it directly references or indicates the department_id from the outer query (e.department_id).
- A correlated subquery is executed once for each row processed by the outer query.
Main Differences Between Correlated and Non-Correlated Subqueries in sql
| Aspect of query | Non-Correlated Subquery features | Correlated Subquery features |
| Execution type | Here non-correlated subquery must Executed once for the entire outer query in given table statement. | Here corelated subquery Executed once for each row of the table outer query in database table until it doesn’t find its all matches. |
| Dependency on Outer Query or not | non-correlated subquery No dependency on the outer query or database it must run. | corelated subquery complete depend or References columns from the outer query table statement. |
| Each Performance | non-correlated subquery, more efficient and faster than other method. | corelated subquery, less efficient or slow in execution process due to multiple individual table row executions for each row. |
| Where to Use | non-correlated subquery Used when the result of the subquery doesn’t depend on the outer query statement. | corelated subquery Used when the subquery needs to access data from each row of the outer query statement. |
| Illustration | non-correlated subquery used to Find employee whose salary is greater than the average salary of all employee list in employee database table. | corelated subquery used to Find employee whose salary is greater than the average salary in their complete department employee. |
Conclusion on Correlated vs. Non-Correlated Subqueries.
- Non-correlated subqueries – are completely independent, efficient, fast, and execute only once for the outer query. Non-correlated subqueries are ideal and effective for certain conditions in the database that do not depend entirely on the outer query.
- Correlated subqueries – on the other hand, are completely dependent on the outer query. They execute once for each row in the database table, and because they execute repeatedly for every row in the database table, the process is somewhat slower.
