GROUPING SETS for multiple groupings in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of GROUPING SETS in SQL?

GROUPING SETS lets you group data by multiple combinations of columns in one query. It helps you get several groupings without writing many queries.

beginner

How does GROUPING SETS differ from using multiple GROUP BY queries?

GROUPING SETS combines multiple grouping queries into one. This saves time and makes the query easier to read and faster to run.

intermediate

Write a simple example of GROUPING SETS to group sales by region and product separately.

SELECT region, product, SUM(sales) FROM sales_data GROUP BY GROUPING SETS ((region), (product));

intermediate

What does the GROUPING() function do when used with GROUPING SETS?

GROUPING() tells you if a column is aggregated or not in the current row. It returns 1 if the column is not grouped (aggregated), 0 if it is grouped.

advanced

Can GROUPING SETS be combined with other grouping methods like ROLLUP or CUBE?

Yes, GROUPING SETS can be combined with ROLLUP and CUBE to create complex grouping combinations in one query.

What does GROUPING SETS ((a), (b)) do in a query?

A Joins tables a and b

B Groups data by both columns a and b together

C Filters rows where a equals b

D Groups data by column a and by column b separately

GROUPING SETS ((a), (b)) creates two separate groups: one by column a and one by column b.

Which SQL clause is used to specify multiple grouping sets?

A GROUP BY GROUPING SETS

B ORDER BY GROUPING SETS

C WHERE GROUPING SETS

D HAVING GROUPING SETS

The GROUP BY GROUPING SETS clause defines multiple grouping sets in one query.

What value does GROUPING(column) return if the column is included in the grouping?

A 1

B 0

C NULL

D -1

GROUPING(column) returns 0 if the column is grouped (not aggregated).

Which of these is NOT a valid grouping method in SQL?

A GROUPING SETS

B ROLLUP

C FILTER

D CUBE

FILTER is not a grouping method; GROUPING SETS, ROLLUP, and CUBE are valid grouping methods.

Why use GROUPING SETS instead of multiple separate queries?

A To combine multiple groupings in one query for efficiency

B To sort data faster

C To filter rows by multiple conditions

D To join tables automatically

GROUPING SETS combines multiple grouping queries into one, making it more efficient.

Explain how GROUPING SETS helps when you want to group data by different column combinations in one query.

Think about grouping sales by region and product separately in one query.

Describe the role of the GROUPING() function when used with GROUPING SETS.

It helps tell if a NULL means no data or aggregated data.

Practice

1. What is the main purpose of using GROUPING SETS in a PostgreSQL query?

easy

A. To group data by multiple combinations in a single query

B. To filter rows based on conditions

C. To join multiple tables together

D. To sort the result set by multiple columns

Solution

1. Step 1: Understand GROUPING SETS functionality

   GROUPING SETS allows grouping by multiple sets of columns in one query, combining results.

2. Step 2: Compare with other options

   Filtering rows is done by WHERE, joining tables by JOIN, sorting by ORDER BY, not GROUPING SETS.

3. Final Answer:

   To group data by multiple combinations in a single query -> Option A

4. Quick Check:

   GROUPING SETS = multiple groupings in one query [OK]

Hint: GROUPING SETS groups multiple ways at once [OK]

Common Mistakes:

• Confusing GROUPING SETS with WHERE filtering
• Thinking GROUPING SETS sorts data
• Mixing GROUPING SETS with JOIN operations

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

easy

A. SELECT col1, col2 FROM table ORDER BY GROUPING SETS (col1, col2);

B. SELECT col1, col2 FROM table WHERE GROUPING SETS (col1, col2);

C. SELECT col1, col2 FROM table HAVING GROUPING SETS (col1, col2);

D. SELECT col1, col2 FROM table GROUP BY GROUPING SETS ((col1), (col2));

Solution

1. Step 1: Identify correct GROUP BY usage

   GROUPING SETS is used inside GROUP BY clause with sets of columns in parentheses.

2. Step 2: Check other clauses

   WHERE, ORDER BY, HAVING do not support GROUPING SETS syntax.

3. Final Answer:

   SELECT col1, col2 FROM table GROUP BY GROUPING SETS ((col1), (col2)); -> Option D

4. Quick Check:

   GROUPING SETS goes inside GROUP BY [OK]

Hint: GROUPING SETS always follows GROUP BY keyword [OK]

Common Mistakes:

• Placing GROUPING SETS in WHERE clause
• Using GROUPING SETS with ORDER BY
• Confusing HAVING with GROUP BY for GROUPING SETS

3. Given the table sales(product, region, amount), what will be the output of this query?

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

medium

A. Sum of amounts grouped by product only

B. Sum of amounts grouped by product and region combined

C. Sum of amounts grouped by product and sum grouped by region in one result

D. Syntax error due to wrong GROUPING SETS usage

Solution

1. Step 1: Understand GROUPING SETS ((product), (region))

   This groups data twice: once by product alone, once by region alone, producing two sets of grouped sums.

2. Step 2: Analyze output rows

   Result will have rows with product and NULL region, and rows with region and NULL product, each showing sum of amounts.

3. Final Answer:

   Sum of amounts grouped by product and sum grouped by region in one result -> Option C

4. Quick Check:

   GROUPING SETS gives multiple groupings combined [OK]

Hint: GROUPING SETS returns all specified groupings combined [OK]

Common Mistakes:

• Expecting grouping by both columns together
• Thinking it returns only one grouping
• Assuming syntax error without checking GROUP BY clause

4. Identify the error in this query:

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

medium

A. Missing WHERE clause for filtering

B. GROUPING SETS requires each grouping to be enclosed in parentheses

C. GROUP BY cannot have multiple columns

D. SUM() cannot be used with GROUPING SETS

Solution

1. Step 1: Check GROUPING SETS syntax

   GROUPING SETS expects a list of grouping sets, each enclosed in parentheses, e.g., ((product), (region)).

2. Step 2: Identify the error

   The query uses (product, region) without extra parentheses, which is invalid syntax for GROUPING SETS.

3. Final Answer:

   GROUPING SETS requires each grouping to be enclosed in parentheses -> Option B

4. Quick Check:

   GROUPING SETS syntax needs nested parentheses [OK]

Hint: Each grouping set must be in its own parentheses [OK]

Common Mistakes:

• Omitting parentheses around grouping sets
• Thinking SUM() is incompatible with GROUPING SETS
• Confusing GROUP BY multiple columns with GROUPING SETS syntax

5. You want to create a report showing total sales grouped by product, by region, and by both product and region together. Which GROUPING SETS clause achieves this in one query?

hard

A. GROUP BY GROUPING SETS ((product), (region), (product, region))

B. GROUP BY product, region

C. GROUP BY GROUPING SETS (product, region)

D. GROUP BY CUBE (product, region)

Solution

1. Step 1: Understand required groupings

   We need three groupings: by product alone, by region alone, and by both together.

2. Step 2: Match with GROUPING SETS syntax

   GROUPING SETS ((product), (region), (product, region)) explicitly lists all three groupings.

3. Step 3: Evaluate other options

   GROUP BY product, region groups by both columns together only; C is invalid syntax; D (CUBE) includes all combinations but is not GROUPING SETS.

4. Final Answer:

   GROUP BY GROUPING SETS ((product), (region), (product, region)) -> Option A

5. Quick Check:

   Explicit sets list all needed groupings [OK]

Hint: List all groupings inside GROUPING SETS parentheses [OK]

Common Mistakes:

• Using GROUP BY without GROUPING SETS for multiple groupings
• Confusing CUBE with GROUPING SETS
• Incorrect syntax without nested parentheses