MySQL ROW_NUMBER()

The MySQL ROW_NUMBER() function is used to assign a unique integer value to each row within a result set. This value is often used to create row numbers or rankings for the rows returned by a query. The ROW_NUMBER() function is typically used in conjunction with the OVER() clause to define the order in which rows are numbered.

Here’s the basic syntax of the ROW_NUMBER() function:

ROW_NUMBER() OVER (PARTITION BY partition_expression ORDER BY sort_expression)

• PARTITION BY: An optional clause that divides the result set into partitions to calculate row numbers separately within each partition.
• ORDER BY: Specifies the column or columns by which the rows are ordered to determine the row numbers.

Here’s an example of how to use the ROW_NUMBER() function:

Suppose you have a table named employees with columns employee_id and salary, and you want to assign a row number to each employee based on their salary, ordered in descending order within each department:

SELECT 
    employee_id, 
    salary, 
    ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS row_num
FROM employees;

In this example, the ROW_NUMBER() function assigns a unique row number to each employee within their respective department based on their salary in descending order.

The result would look something like this:

employee_id | salary | row_num
----------- | ------ | -------
1          | 60000  | 1
2          | 55000  | 2
3          | 62000  | 1
4          | 58000  | 2
5          | 65000  | 1
6          | 59000  | 2

You can use the row numbers for various purposes, such as ranking rows, implementing pagination, or identifying duplicates. The ROW_NUMBER() function is a valuable tool for such tasks in MySQL.