Finding the Nth Highest Salary

Finding the Nth Highest Salary

In many database applications, you might need to retrieve the Nth highest value from a table. This is a common task in reporting, analytics, and data mining. This tutorial explains different methods to efficiently find the Nth highest salary (or any Nth highest value) in a database table.

Scenario:

Let’s assume we have a table named Employee with the following columns:

• EmpID (Employee ID)
• EmpName (Employee Name)
• EmpSalary (Employee Salary)

We want to find the 3rd highest salary, or more generally, the Nth highest salary, from this table.

Methods to Find the Nth Highest Salary:

Here are several approaches to solve this problem, with explanations and example SQL queries.

1. Using ROW_NUMBER() Window Function

Window functions are a powerful tool for assigning ranks to rows within a result set. ROW_NUMBER() assigns a unique sequential integer to each row within a partition, based on the specified ordering.

WITH RankedSalaries AS (
    SELECT 
        EmpID,
        EmpName,
        EmpSalary,
        ROW_NUMBER() OVER (ORDER BY EmpSalary DESC) AS RowNum
    FROM 
        Employee
)
SELECT 
    EmpID,
    EmpName,
    EmpSalary
FROM 
    RankedSalaries
WHERE 
    RowNum = @NthRow; -- Replace @NthRow with the desired N value (e.g., 3 for the 3rd highest)

Explanation:

1. The WITH clause defines a Common Table Expression (CTE) called RankedSalaries.
2. Inside the CTE, ROW_NUMBER() OVER (ORDER BY EmpSalary DESC) assigns a rank to each employee based on their salary in descending order. The highest salary gets rank 1, the second highest gets rank 2, and so on.
3. The final SELECT statement retrieves the employee details where the RowNum is equal to the desired Nth value.

2. Using DENSE_RANK() Window Function

Similar to ROW_NUMBER(), DENSE_RANK() assigns ranks to rows. However, DENSE_RANK() assigns the same rank to rows with the same value. If multiple employees have the same salary, they will receive the same rank, and the next distinct salary will receive the subsequent rank without skipping.

WITH RankedSalaries AS (
    SELECT 
        EmpID,
        EmpName,
        EmpSalary,
        DENSE_RANK() OVER (ORDER BY EmpSalary DESC) AS RankNum
    FROM 
        Employee
)
SELECT 
    EmpID,
    EmpName,
    EmpSalary
FROM 
    RankedSalaries
WHERE 
    RankNum = @NthRow;

The difference between ROW_NUMBER() and DENSE_RANK() is crucial when dealing with duplicate salary values. If you need to find the Nth distinct salary, DENSE_RANK() is preferred.

3. Using a Subquery and LIMIT (MySQL, PostgreSQL)

For some database systems (like MySQL or PostgreSQL), you can use a subquery combined with LIMIT to achieve the same result.

SELECT MIN(EmpSalary)
FROM (
    SELECT EmpSalary
    FROM Employee
    ORDER BY EmpSalary DESC
    LIMIT @NthRow
) AS TopSalaries;

Explanation:

1. The subquery SELECT EmpSalary FROM Employee ORDER BY EmpSalary DESC LIMIT @NthRow retrieves the top N salaries.
2. The outer query SELECT MIN(EmpSalary) FROM ... then selects the minimum salary from those top N salaries, which effectively gives you the Nth highest salary.

4. Using TOP (SQL Server)

SQL Server provides the TOP keyword to select a limited number of rows. You can combine it with an ORDER BY clause to retrieve the top N salaries and then select the minimum from that result.

SELECT MIN(EmpSalary)
FROM (
    SELECT TOP (@NthRow) EmpSalary
    FROM Employee
    ORDER BY EmpSalary DESC
) AS TopSalaries;

Explanation:

This query functions similarly to the LIMIT approach but uses SQL Server’s TOP keyword.

Choosing the Right Method:

• For general use and portability across different database systems, the ROW_NUMBER() or DENSE_RANK() window function approach is often preferred.
• The LIMIT and TOP approaches can be more concise for certain database systems but might not be as portable.
• Consider the possibility of duplicate salaries and choose between ROW_NUMBER() and DENSE_RANK() accordingly.