MySQL ROW_NUMBER, This is How You Emulate It

Emulating MySQL ROW_NUMBER Function

MySQL versions before 8.0 did not directly support the

window function. However, starting with MySQL 8.0, the ROW_NUMBER() function is natively supported, making it easier to assign unique row numbers within partitions of a result set.

For earlier MySQL versions, you can emulate the behavior of ROW_NUMBER() using user-defined variables. This guide will explain the native ROW_NUMBER() function and the method to emulate it.

1. Using the ROW_NUMBER() Function (MySQL 8.0 and Later)

Syntax

ROW_NUMBER() OVER (
    [PARTITION BY column_name]
    [ORDER BY column_name]
)

PARTITION BY: Divides the result set into partitions. The row numbers are reset for each partition.
ORDER BY: Specifies the order of rows within each partition.

Example 1: Assign Row Numbers to All Rows

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

This query assigns a unique row number to each employee based on their salary in descending order.

Example 2: Assign Row Numbers Within Each Partition

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

This query assigns row numbers to employees within each department, ordered by salary in descending order.

2. Emulating ROW_NUMBER() in MySQL (Before 8.0)

For MySQL versions earlier than 8.0, you can emulate the ROW_NUMBER() function using user-defined variables.

Steps

Declare and initialize a user-defined variable.
Use the variable to assign row numbers in the query.
Optionally reset the variable for each partition using conditions.

Example 1: Assign Row Numbers Without Partitioning

SET @row_number = 0;

SELECT employee_id, department_id, salary,
       (@row_number := @row_number + 1) AS row_number
FROM employees
ORDER BY salary DESC;

This query assigns row numbers to all rows based on their salary in descending order.

Example 2: Assign Row Numbers With Partitioning

To emulate PARTITION BY, you need to reset the row number for each partition.

SET @row_number = 0;
SET @current_department = NULL;

SELECT employee_id, department_id, salary,
       (@row_number := IF(@current_department = department_id, @row_number + 1, 1)) AS row_number,
       (@current_department := department_id)
FROM employees
ORDER BY department_id, salary DESC;

Here’s how it works:

@current_department tracks the current department.
IF(@current_department = department_id, @row_number + 1, 1) resets the row number when the department changes.

Key Differences Between Native and Emulated ROW_NUMBER()

Feature	Native ROW_NUMBER()	Emulated ROW_NUMBER()
Simplicity	Easy to use	Requires manual variable handling
Performance	Optimized by MySQL	May impact performance on large datasets
Partition Support	Built-in with `PARTITION BY`	Manually handled with conditions

Common Use Cases for ROW_NUMBER()

Pagination: Assign row numbers to implement data pagination in web applications.

SELECT *
FROM (
    SELECT employee_id, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS row_number
    FROM employees
) AS ranked
WHERE row_number BETWEEN 1 AND 10;

Ranked Queries: Generate rankings for employees, products, or other entities.

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

Deduplication: Remove duplicate rows by keeping only the first occurrence.

SELECT *
FROM (
    SELECT employee_id, ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY join_date) AS row_number
    FROM employees
) AS ranked
WHERE row_number = 1;

Conclusion

For MySQL 8.0 and later: Use the native ROW_NUMBER() function for simplicity and performance.
For earlier MySQL versions: Emulate ROW_NUMBER() using user-defined variables. While more complex, it is effective for achieving similar results.

Understanding both methods ensures you can handle scenarios where you need row numbering, regardless of the MySQL version.

Souy Soeng