ROW_NUMBER

Returns a unique row number for each row within a window partition.

The row number starts at 1 and continues up sequentially.

Syntax

ROW_NUMBER() OVER (
  [ PARTITION BY <expr1> [, <expr2> ... ] ]
  ORDER BY <expr3> [ , <expr4> ... ] [ { ASC | DESC } ]
  )

Examples

Suppose we have a table called employees with columns employee_id, first_name, last_name, department, and salary.

We want to number employees within each department based on their salaries in descending order.

Create the table

CREATE TABLE employees (
  employee_id INT,
  first_name VARCHAR,
  last_name VARCHAR,
  department VARCHAR,
  salary INT
);

Insert data

INSERT INTO employees (employee_id, first_name, last_name, department, salary) VALUES
  (1, 'John', 'Doe', 'IT', 90000),
  (2, 'Jane', 'Smith', 'HR', 85000),
  (3, 'Mike', 'Johnson', 'IT', 82000),
  (4, 'Sara', 'Williams', 'Sales', 77000),
  (5, 'Tom', 'Brown', 'HR', 75000);

Numbering employees within departments

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

Result:

employee_id	first_name	last_name	department	salary	row_num
1	John	Doe	IT	90000	1
3	Mike	Johnson	IT	82000	2
2	Jane	Smith	HR	85000	1
5	Tom	Brown	HR	75000	2
4	Sara	Williams	Sales	77000	1