ROLLUP and CUBE for hierarchical totals in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate hierarchical totals using ROLLUP.

PostgreSQL

SELECT department, product, SUM(sales) FROM sales_data GROUP BY [1];

Drag options to blanks, or click blank then click option'

ACUBE(department, product)

BGROUPING SETS(department, product)

CROLLUP(department, product)

DGROUP BY department, product

Attempts:

3 left

💡 Hint

Common Mistakes

Using CUBE instead of ROLLUP when hierarchical totals are needed.

Forgetting to use an aggregate function like SUM.

The ROLLUP function creates hierarchical totals by grouping first by department, then product, and finally a grand total.

2fill in blank

medium

Complete the code to calculate all combinations of totals using CUBE.

PostgreSQL

SELECT region, category, SUM(amount) FROM sales GROUP BY [1];

Drag options to blanks, or click blank then click option'

ACUBE(region, category)

BGROUPING SETS((region), (category))

CROLLUP(region, category)

DGROUP BY region, category

Attempts:

3 left

💡 Hint

Common Mistakes

Using ROLLUP instead of CUBE when all combinations are needed.

Not including all grouping columns inside the CUBE function.

The CUBE function generates all possible combinations of grouping columns, giving totals for each combination.

3fill in blank

hard

Fix the error in the GROUP BY clause to correctly use ROLLUP.

PostgreSQL

SELECT category, subcategory, SUM(revenue) FROM sales_data GROUP BY [1];

Drag options to blanks, or click blank then click option'

AGROUPING SETS(category, subcategory)

BCUBE(category, subcategory)

Ccategory, subcategory

DROLLUP(category, subcategory)

Attempts:

3 left

💡 Hint

Common Mistakes

Listing columns without ROLLUP or CUBE for hierarchical totals.

Using GROUPING SETS incorrectly without parentheses.

To get hierarchical totals, the GROUP BY clause must use ROLLUP(category, subcategory) instead of just listing columns.

4fill in blank

hard

Fill both blanks to create a query that uses CUBE and filters out grand totals.

PostgreSQL

SELECT region, product, SUM(sales) FROM sales_data GROUP BY [1] HAVING GROUPING([2]) = 0;

Drag options to blanks, or click blank then click option'

ACUBE(region, product)

Bregion

Cproduct

DROLLUP(region, product)

Attempts:

3 left

💡 Hint

Common Mistakes

Using ROLLUP instead of CUBE when all combinations are needed.

Filtering on the wrong column in GROUPING.

Using CUBE(region, product) groups all combinations. The GROUPING(region) = 0 condition filters out rows where region is a subtotal or grand total.

5fill in blank

hard

Fill all three blanks to write a query that uses ROLLUP and shows which rows are subtotals or totals.

PostgreSQL

SELECT department, team, SUM(profit), GROUPING([1]) AS dept_grouping, GROUPING([2]) AS team_grouping FROM profits GROUP BY [3];

Drag options to blanks, or click blank then click option'

Adepartment

Bteam

CROLLUP(department, team)

DCUBE(department, team)

Attempts:

3 left

💡 Hint

Common Mistakes

Using CUBE instead of ROLLUP when hierarchical totals are needed.

Mismatching columns inside GROUPING and GROUP BY.

This query uses ROLLUP(department, team) to get hierarchical totals. The GROUPING() function shows if a row is a subtotal or total for each column.

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