MySQL

In MySQL, ranking rows within groups can be achieved using various techniques, such as window functions (ROW_NUMBER(), RANK(), DENSE_RANK()) or manual ranking with subqueries and GROUP BY. Group ranking is useful when you want to assign a rank to records within a specific group, such as ranking employees based on salary within each department.

Here’s how to perform group ranking in MySQL, along with examples and useful tips for new students:

1. Using ROW_NUMBER() for Group Ranking:

   • The ROW_NUMBER() function assigns a unique sequential integer to rows within a result set, starting from 1 for each group. It’s commonly used for ranking rows based on a specific order within groups.
   • Syntax:

   SELECT 
     first_name, last_name, department_id, salary, 
     ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank
   FROM employees;

   • This query ranks employees by salary within each department. The PARTITION BY clause creates separate ranking groups for each department_id.

2. Using RANK() for Group Ranking:

   • The RANK() function assigns ranks to rows within groups, but it gives the same rank to rows with the same value. The next rank is skipped when there is a tie.
   • Example:

   SELECT 
     first_name, last_name, department_id, salary, 
     RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank
   FROM employees;

   • This query ranks employees by salary within their department. If two employees have the same salary, they will receive the same rank, and the next rank will be skipped.

3. Using DENSE_RANK() for Group Ranking:

   • The DENSE_RANK() function is similar to RANK(), but it does not skip ranks when there are ties.
   • Example:

   SELECT 
     first_name, last_name, department_id, salary, 
     DENSE_RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank
   FROM employees;

   • This query assigns ranks based on salary within each department, but ranks are not skipped if there are ties.

4. Manual Ranking Using Subqueries:

   • If your MySQL version does not support window functions (prior to MySQL 8.0), you can manually create ranks using subqueries.
   • Example:

   SELECT e.first_name, e.last_name, e.department_id, e.salary, 
     (SELECT COUNT(*) + 1 
      FROM employees e2 
      WHERE e2.salary > e.salary 
      AND e2.department_id = e.department_id) AS rank
   FROM employees e;

   • This query ranks employees by salary within their department by counting the number of employees with a higher salary.

5. Using RANK() with Multiple Criteria:

   • You can use multiple columns in the ORDER BY clause for more complex ranking.
   • Example:

   SELECT 
     first_name, last_name, department_id, salary, hire_date, 
     RANK() OVER (PARTITION BY department_id ORDER BY salary DESC, hire_date ASC) AS rank
   FROM employees;

   • This query ranks employees by salary within their department, and in case of ties, the ranking is further sorted by hire date.

6. Ranking with GROUP BY:

   • You can also use GROUP BY along with aggregate functions to create a basic ranking system, though it’s less flexible than window functions.
   • Example:

   SELECT department_id, salary, COUNT(*) AS rank
   FROM employees e1
   JOIN employees e2
   ON e1.department_id = e2.department_id 
   AND e1.salary >= e2.salary
   GROUP BY e1.employee_id;

   • This query ranks employees by salary within each department using a join and group count.

7. Performance Considerations:

   • Window functions like ROW_NUMBER(), RANK(), and DENSE_RANK() are efficient and easier to use than manual ranking techniques. For large datasets, using these built-in functions can improve performance.

8. Combining Ranking with HAVING:

   • You can filter ranked results using the HAVING clause to limit the output to certain ranks.
   • Example:

   SELECT 
     first_name, last_name, department_id, salary, 
     RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank
   FROM employees
   HAVING rank <= 3;

   • This query returns the top 3 highest-paid employees within each department.

Group ranking in MySQL is a powerful way to analyze and categorize data within specific groups. Whether using built-in window functions or manual methods, ranking helps you gain deeper insights into your data.

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SELECT employee_number
    department,
    salary,
    RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS salary_rank
FROM org_employee;

SQL GROUP RANK

Utilize the RANK() function to assign rankings to employee salaries within specific departments.

RAW EMPLOYEE DATA

QUERY DATA MAPPING

This image demonstrates that we need to rank each salary within a specified department.

QUERY OUT PUT

Here you can see ranking of each salary with in a given department.