ROLLUP and CUBE for hierarchical totals in PostgreSQL - Step-by-Step Execution

Concept Flow - ROLLUP and CUBE for hierarchical totals

Start with base data

↓

Apply GROUP BY with ROLLUP or CUBE

↓

Generate grouping sets

↓

Calculate aggregates for each grouping set

↓

Return result with hierarchical totals

ROLLUP and CUBE create multiple levels of grouped totals from base data, showing subtotals and grand totals automatically.

Execution Sample

PostgreSQL

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

This query calculates sales totals by region and product, including subtotals per region and a grand total.

Execution Table

Step	region	product	SUM(sales)	Grouping Level	Action
1	East	Apple	100	region+product	Calculate sum for East & Apple
2	East	Banana	150	region+product	Calculate sum for East & Banana
3	East	-	250	region subtotal	Sum of East region products
4	West	Apple	200	region+product	Calculate sum for West & Apple
5	West	Banana	100	region+product	Calculate sum for West & Banana
6	West	-	300	region subtotal	Sum of West region products
7	-	-	550	grand total	Sum of all sales
8	Exit	No more grouping sets			ROLLUP completed

💡 All grouping sets processed: detailed, subtotals, and grand total

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
region	null	East	East	East	West	West	West	null	null
product	null	Apple	Banana	null	Apple	Banana	null	null	null
SUM(sales)	0	100	150	250	200	100	300	550	550

Key Moments - 3 Insights

Why do some rows have NULL in the product column?

Rows with NULL in product represent subtotals for the region (see rows 3 and 6 in execution_table), showing totals for all products in that region.

What does the row with NULL in both region and product mean?

This row (row 7) is the grand total, summing all sales across all regions and products.

How does ROLLUP differ from a simple GROUP BY?

ROLLUP adds extra grouping sets for subtotals and grand totals automatically, unlike simple GROUP BY which only groups by specified columns (see execution_table for grouping levels).

Visual Quiz - 3 Questions

Look at the execution_table, what is the SUM(sales) for the West region subtotal?

A 200

B 300

C 100

D 550

💡 Hint

Check row 6 in the execution_table where region is West and product is NULL.

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

A Step 1

B Step 3

C Step 7

D Step 5

💡 Hint

Look for the row where both region and product are NULL representing the grand total.

If we replaced ROLLUP with CUBE, how would the number of grouping sets change?

A It would increase

B It would decrease

C It would stay the same

D It would be zero

💡 Hint

CUBE generates all combinations of grouping columns, more than ROLLUP which is hierarchical.

Concept Snapshot

ROLLUP and CUBE add hierarchical totals to GROUP BY queries.
ROLLUP creates subtotals from left to right columns.
CUBE creates all combinations of grouping columns.
NULL in result means subtotal or total level.
Use to get detailed + summary data in one query.

Full Transcript

ROLLUP and CUBE are SQL features that help calculate totals at multiple levels automatically. When you GROUP BY columns with ROLLUP, it adds extra rows for subtotals and a grand total. For example, grouping sales by region and product with ROLLUP shows sales per product, per region subtotal, and overall total. The execution table shows each step: first sums per region and product, then subtotals per region (with NULL product), and finally the grand total (NULL region and product). NULL values in grouping columns indicate these subtotal or total rows. CUBE is similar but creates all possible combinations of grouping columns, resulting in more grouping sets. This helps get detailed and summary data in one query without writing multiple queries.