Window functions (ROW_NUMBER) in MySQL - Step-by-Step Execution

Concept Flow - Window functions (ROW_NUMBER)

Start with table data

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Define window partition/order

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Apply ROW_NUMBER() over window

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Assign row numbers per partition/order

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Return result with row numbers

ROW_NUMBER() assigns a unique number to each row within a partition of the result set, ordered by specified columns.

Execution Sample

MySQL

SELECT name, department,
       ROW_NUMBER() OVER (PARTITION BY department ORDER BY name) AS rn
FROM employees;

Assigns a row number to each employee within their department, ordered by name.

Execution Table

Step	Row Data (name, department)	Partition	Order by name	ROW_NUMBER assigned
1	(Alice, Sales)	Sales	Alice	1
2	(Bob, Sales)	Sales	Bob	2
3	(Charlie, HR)	HR	Charlie	1
4	(David, HR)	HR	David	2
5	(Eve, IT)	IT	Eve	1
6	(Frank, IT)	IT	Frank	2
7	(Grace, IT)	IT	Grace	3
8	(Heidi, Sales)	Sales	Heidi	3
Exit	All rows processed			ROW_NUMBER assigned per partition

💡 All rows processed, each assigned a row number within their department ordered by name.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	Final
ROW_NUMBER (Sales)	0	1	2						3	3
ROW_NUMBER (HR)	0			1	2					2
ROW_NUMBER (IT)	0					1	2	3		3

Key Moments - 3 Insights

Why does ROW_NUMBER restart at 1 for each department?

Because ROW_NUMBER() is used with PARTITION BY department, it resets counting for each department group as shown in execution_table rows 1-2 (Sales), 3-4 (HR), and 5-7 (IT).

What determines the order of row numbers within each partition?

The ORDER BY clause inside OVER() defines the order. Here, rows are ordered by name alphabetically within each department, as seen in the 'Order by name' column.

What happens if we remove PARTITION BY?

ROW_NUMBER() would assign unique numbers across the entire result set without resetting, numbering all rows from 1 to total rows.

Visual Quiz - 3 Questions

Look at the execution_table, what is the ROW_NUMBER assigned to Heidi in Sales?

A 2

B 3

C 1

D 4

💡 Hint

Check the row with Heidi in the execution_table under 'ROW_NUMBER assigned' column.

At which step does the ROW_NUMBER for HR department reach 2?

A Step 3

B Step 5

C Step 4

D Step 2

💡 Hint

Look at execution_table rows for HR partition and their assigned ROW_NUMBER.

If we remove PARTITION BY, how would the ROW_NUMBER for Alice change?

A It would be 1

B It would be 4

C It would be 8

D It would be 0

💡 Hint

Without partition, ROW_NUMBER counts from 1 for the whole table, so first row is 1.

Concept Snapshot

ROW_NUMBER() assigns unique row numbers per partition.
Use PARTITION BY to reset numbering per group.
ORDER BY defines numbering order.
Without PARTITION BY, numbering is global.
Syntax: ROW_NUMBER() OVER (PARTITION BY col ORDER BY col).

Full Transcript

This visual trace shows how the ROW_NUMBER() window function works in MySQL. Starting with a table of employees, the function assigns a unique number to each row within each department group, ordered by employee name. The flow begins by defining partitions by department and ordering by name. Then, ROW_NUMBER() counts rows starting at 1 for each department. The execution table shows each row processed with its assigned number. Variables track row numbers per department as they increment. Key moments clarify why numbering restarts per partition and how ordering affects results. The quiz tests understanding of row numbers assigned at specific steps and the effect of removing partitioning. The snapshot summarizes syntax and behavior for quick reference.