ROLLUP for subtotals in MySQL - Step-by-Step Execution

Concept Flow - ROLLUP for subtotals

Start with GROUP BY columns

↓

Aggregate rows by groups

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Add subtotal rows for each grouping level

↓

Add grand total row

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Return result with subtotals and total

ROLLUP groups rows and adds extra rows for subtotals and a grand total automatically.

Execution Sample

MySQL

SELECT category, product, SUM(sales) AS total_sales
FROM sales_data
GROUP BY category, product WITH ROLLUP;

This query sums sales by category and product, then adds subtotal rows for each category and a grand total.

Execution Table

Step	category	product	SUM(sales)	Explanation
1	Electronics	Phone	1000	Sum sales for Electronics Phone
2	Electronics	Laptop	1500	Sum sales for Electronics Laptop
3	Electronics	-	2500	Subtotal for Electronics category (Phone + Laptop)
4	Furniture	Chair	700	Sum sales for Furniture Chair
5	Furniture	Table	300	Sum sales for Furniture Table
6	Furniture	-	1000	Subtotal for Furniture category (Chair + Table)
7	-	-	3500	Grand total for all categories and products

💡 All groups processed, subtotals and grand total rows added by ROLLUP

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
category	null	Electronics	Electronics	Electronics	Furniture	Furniture	Furniture	null
product	null	Phone	Laptop	null	Chair	Table	null	null
SUM(sales)	0	1000	1500	2500	700	300	1000	3500

Key Moments - 3 Insights

Why does the product column show null in some rows?

Rows with null in product are subtotal rows for the category, as shown in execution_table rows 3 and 6.

What does the row with both category and product as null represent?

That row is the grand total for all categories and products, shown in execution_table row 7.

How does ROLLUP decide which subtotal rows to add?

ROLLUP adds subtotal rows by progressively removing columns from right to left in the GROUP BY, as seen in the subtotal rows with null product.

Visual Quiz - 3 Questions

Look at the execution_table, what is the SUM(sales) value at step 3?

A 2500

B 1500

C 1000

D 3500

💡 Hint

Check the row where category is Electronics and product is null in execution_table.

At which step does the grand total appear in the execution_table?

A Step 6

B Step 7

C Step 3

D Step 1

💡 Hint

Look for the row where both category and product are null.

If we remove product from GROUP BY and use ROLLUP on category only, how would the subtotal rows change?

A There would be no subtotal rows

B Subtotal rows would be for product only

C Subtotal rows would be for category only, no product breakdown

D Grand total row would be removed

💡 Hint

ROLLUP adds subtotals by removing columns from right to left in GROUP BY.

Concept Snapshot

ROLLUP adds subtotal and total rows automatically.
Use WITH ROLLUP after GROUP BY.
It groups by all columns, then adds subtotals by removing columns from right.
Nulls in grouped columns mark subtotal or total rows.
Useful for quick summary reports.

Full Transcript

ROLLUP is a SQL feature that helps you get subtotals and grand totals easily. When you group data by columns and add WITH ROLLUP, the database first groups rows normally. Then it adds extra rows that show subtotals for each group level by replacing the rightmost grouped columns with null. Finally, it adds a grand total row with all grouped columns as null. For example, grouping sales by category and product with ROLLUP will show sales per product, subtotals per category, and a grand total. Null values in grouped columns indicate subtotal or total rows. This makes reports easier to read and saves writing multiple queries.