ROLLUP and CUBE for hierarchical totals in PostgreSQL

Introduction

ROLLUP and CUBE help you quickly get totals and subtotals for groups of data. They make it easy to see summaries at different levels.

You want to see sales totals by region, then by country within each region, and then overall total.

You need to analyze expenses by department and project, including subtotals and grand totals.

You want to create reports that show data summaries at multiple levels without writing many queries.

You want to compare combinations of categories and see all possible totals in one result.

Syntax

PostgreSQL

SELECT columns, AGGREGATE_FUNCTION(column)
FROM table
GROUP BY ROLLUP(column1, column2, ...);

-- or

SELECT columns, AGGREGATE_FUNCTION(column)
FROM table
GROUP BY CUBE(column1, column2, ...);

ROLLUP creates subtotals moving from left to right in the list of columns, ending with a grand total.

CUBE creates subtotals for all combinations of the listed columns, including the grand total.

Examples

This query shows sales totals by region, then by country within each region, and a grand total.

PostgreSQL

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

This query shows costs for every combination of department and project, plus subtotals and grand total.

PostgreSQL

SELECT department, project, SUM(cost) AS total_cost
FROM expenses
GROUP BY CUBE(department, project);

Sample Program

This example creates a sales table, inserts sample data, and uses ROLLUP to get totals by region, country, and grand total.

PostgreSQL

CREATE TABLE sales (
  region TEXT,
  country TEXT,
  sales INT
);

INSERT INTO sales VALUES
('North America', 'USA', 100),
('North America', 'Canada', 150),
('Europe', 'France', 200),
('Europe', 'Germany', 250);

SELECT region, country, SUM(sales) AS total_sales
FROM sales
GROUP BY ROLLUP(region, country)
ORDER BY region NULLS LAST, country NULLS LAST;

Important Notes

NULL values in the result represent subtotal or grand total rows.

Use ORDER BY with NULLS LAST to keep totals at the bottom for better readability.

ROLLUP is good for hierarchical totals; CUBE is better when you want all combinations.

Summary

ROLLUP and CUBE help create subtotals and grand totals in one query.

ROLLUP gives hierarchical totals from left to right.

CUBE gives all combinations of totals for the listed columns.

Practice

1. What does the ROLLUP operator do in a PostgreSQL GROUP BY clause?

easy

A. It filters rows based on a condition before grouping.

B. It creates hierarchical subtotals from left to right for the grouped columns.

C. It generates all possible combinations of grouped columns for totals.

D. It sorts the result set after grouping.

Solution

1. Step 1: Understand ROLLUP purpose

   ROLLUP creates subtotals that roll up from the first column to the last in the GROUP BY list.

2. Step 2: Compare with other options

   CUBE creates all combinations, filtering and sorting are unrelated to ROLLUP.

3. Final Answer:

   It creates hierarchical subtotals from left to right for the grouped columns. -> Option B

4. Quick Check:

   ROLLUP = hierarchical subtotals [OK]

Hint: ROLLUP = subtotals in order of columns [OK]

Common Mistakes:

• Confusing ROLLUP with CUBE
• Thinking ROLLUP filters rows
• Assuming ROLLUP sorts results

2. Which of the following is the correct syntax to use CUBE in a PostgreSQL query?

easy

A. SELECT col1, col2, SUM(sales) FROM table CUBE BY (col1, col2);

B. SELECT col1, col2, SUM(sales) FROM table GROUP BY CUBE col1, col2;

C. SELECT col1, col2, SUM(sales) FROM table GROUP BY ROLLUP(col1, col2);

D. SELECT col1, col2, SUM(sales) FROM table GROUP BY CUBE(col1, col2);

Solution

1. Step 1: Recall correct GROUP BY syntax with CUBE

   The correct syntax is GROUP BY CUBE(col1, col2) with parentheses around columns.

2. Step 2: Identify incorrect options

   SELECT col1, col2, SUM(sales) FROM table CUBE BY (col1, col2); uses invalid 'CUBE BY', C uses ROLLUP, D misses parentheses.

3. Final Answer:

   SELECT col1, col2, SUM(sales) FROM table GROUP BY CUBE(col1, col2); -> Option D

4. Quick Check:

   GROUP BY CUBE(columns) syntax [OK]

Hint: Use GROUP BY CUBE(col1, col2) with parentheses [OK]

Common Mistakes:

• Omitting parentheses in CUBE
• Using 'CUBE BY' instead of 'GROUP BY CUBE'
• Confusing ROLLUP and CUBE syntax

3. Given the table sales with columns region, product, and amount, what will be the output rows count of this query?

SELECT region, product, SUM(amount) FROM sales GROUP BY ROLLUP(region, product);

medium

A. Number of distinct (region, product) pairs plus one subtotal per region plus one grand total row.

B. Number of distinct (region, product) pairs only.

C. Number of distinct regions only.

D. Number of distinct products only.

Solution

1. Step 1: Understand ROLLUP output rows

   ROLLUP(region, product) returns all (region, product) groups, plus subtotals per region, plus a grand total row.

2. Step 2: Count rows

   Total rows = distinct (region, product) + distinct regions (subtotals) + 1 (grand total).

3. Final Answer:

   Number of distinct (region, product) pairs plus one subtotal per region plus one grand total row. -> Option A

4. Quick Check:

   ROLLUP adds subtotals and grand total [OK]

Hint: ROLLUP adds subtotals and grand total rows [OK]

Common Mistakes:

• Ignoring subtotal and grand total rows
• Counting only distinct pairs
• Confusing ROLLUP with CUBE output

4. Identify the error in this query using CUBE:

SELECT region, product, SUM(amount) FROM sales GROUP BY CUBE region, product;

medium

A. GROUP BY cannot use CUBE with more than one column.

B. SUM(amount) cannot be used with CUBE.

C. Missing parentheses around columns in CUBE function.

D. region and product must be in ORDER BY, not GROUP BY.

Solution

1. Step 1: Check CUBE syntax

   CUBE requires parentheses around the columns: CUBE(region, product).

2. Step 2: Validate other parts

   SUM(amount) is valid, multiple columns allowed, ORDER BY is unrelated.

3. Final Answer:

   Missing parentheses around columns in CUBE function. -> Option C

4. Quick Check:

   CUBE syntax needs parentheses [OK]

Hint: Always use parentheses with CUBE columns [OK]

Common Mistakes:

• Omitting parentheses in CUBE
• Thinking SUM is invalid with CUBE
• Confusing GROUP BY and ORDER BY

5. You want to generate a report showing sales totals by region, product, and year, including all subtotals and grand totals. Which query correctly uses CUBE to achieve this?

hard

A. SELECT region, product, year, SUM(amount) FROM sales GROUP BY CUBE(region, product, year);

B. SELECT region, product, year, SUM(amount) FROM sales GROUP BY region, product, year;

C. SELECT region, product, year, SUM(amount) FROM sales GROUP BY GROUPING SETS(region, product, year);

D. SELECT region, product, year, SUM(amount) FROM sales GROUP BY ROLLUP(region, product, year);

Solution

1. Step 1: Understand the requirement

   All subtotals and grand totals for region, product, and year means all combinations of these columns.

2. Step 2: Choose correct grouping method

   CUBE generates all combinations, ROLLUP only hierarchical totals, GROUPING SETS needs explicit sets, simple GROUP BY no subtotals.

3. Final Answer:

   SELECT region, product, year, SUM(amount) FROM sales GROUP BY CUBE(region, product, year); -> Option A

4. Quick Check:

   CUBE = all combinations subtotals [OK]

Hint: Use CUBE for all subtotal combinations [OK]

Common Mistakes:

• Using ROLLUP instead of CUBE for all subtotals
• Confusing GROUPING SETS with CUBE
• Forgetting subtotals with simple GROUP BY