Pivot and unpivot concepts in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a simple PIVOT query

Given the table Sales with columns Year, Product, and Amount, what is the output of this PIVOT query?

SELECT * FROM (
  SELECT Year, Product, Amount FROM Sales
) AS SourceTable
PIVOT (
  SUM(Amount) FOR Product IN ([A], [B])
) AS PivotTable
ORDER BY Year;

SQL

CREATE TABLE Sales (Year INT, Product VARCHAR(10), Amount INT);
INSERT INTO Sales VALUES
(2020, 'A', 100),
(2020, 'B', 150),
(2021, 'A', 200),
(2021, 'B', 250);

A

Year | Product | Amount
-----|---------|-------
2020 | A       | 100
2020 | B       | 150
2021 | A       | 200
2021 | B       | 250

B

Year | A   | B
-----|-----|----
2020 | 150 | 100
2021 | 250 | 200

C

Year | A   | B
-----|-----|----
2020 | NULL| NULL
2021 | NULL| NULL

D

Year | A   | B
-----|-----|----
2020 | 100 | 150
2021 | 200 | 250

💡 Hint

Remember that PIVOT aggregates values and arranges them as columns based on the specified IN list.

The PIVOT query sums Amounts for each Product per Year, turning Product values into columns A and B with their sums.

❓ query_result

intermediate

Output of an UNPIVOT query

Given the table MonthlySales with columns Year, Jan, Feb, what is the output of this UNPIVOT query?

SELECT Year, Month, Sales FROM MonthlySales
UNPIVOT (
  Sales FOR Month IN (Jan, Feb)
) AS UnpivotTable
ORDER BY Year, Month;

SQL

CREATE TABLE MonthlySales (Year INT, Jan INT, Feb INT);
INSERT INTO MonthlySales VALUES
(2020, 100, 150),
(2021, 200, 250);

A

Year | Jan | Feb
-----|-----|----
2020 | 100 | 150
2021 | 200 | 250

B

Year | Month | Sales
-----|-------|------
2020 | Jan   | 100
2020 | Feb   | 150
2021 | Jan   | 200
2021 | Feb   | 250

C

Year | Month | Sales
-----|-------|------
2020 | Jan   | 150
2020 | Feb   | 100
2021 | Jan   | 250
2021 | Feb   | 200

D

Year | Month | Sales
-----|-------|------
2020 | NULL  | NULL
2021 | NULL  | NULL

💡 Hint

UNPIVOT converts columns into rows, so each month becomes a row with its sales.

The UNPIVOT query transforms the Jan and Feb columns into rows under Month with corresponding Sales values.

🧠 Conceptual

advanced

Understanding PIVOT aggregation behavior

Why does a PIVOT operation in SQL require an aggregation function like SUM or COUNT?

A Because PIVOT combines multiple rows into one, aggregation summarizes values for each new column.

B Because PIVOT only works on single rows and does not need aggregation.

C Because aggregation is optional and only used for sorting results.

D Because PIVOT duplicates rows without changing data, so aggregation is irrelevant.

💡 Hint

Think about what happens when multiple rows share the same pivot column value.

PIVOT groups rows by the pivot column and aggregates values to produce one row per group with summarized columns.

📝 Syntax

advanced

Identify the syntax error in this PIVOT query

Which option contains a syntax error in this PIVOT query?

SELECT * FROM (
  SELECT Year, Product, Amount FROM Sales
) AS SourceTable
PIVOT (
  SUM(Amount) FOR Product IN (A, B)
) AS PivotTable;

A SUM(Amount) should be replaced with COUNT(Amount)

B The alias PivotTable is missing after PIVOT keyword

C The IN list should have column names in square brackets: [A], [B]

D The SELECT * should specify columns explicitly

💡 Hint

Check the syntax for the IN list in PIVOT clause.

In SQL Server PIVOT, the IN list must use square brackets around column names to be valid identifiers.

❓ optimization

expert

Optimizing UNPIVOT for large datasets

You have a large table with many monthly sales columns (Jan to Dec). Which approach optimizes performance when unpivoting these columns into rows?

A Use CROSS APPLY with VALUES clause to unpivot columns efficiently

B Use multiple UNION ALL queries to select each month separately and combine

C Create a normalized table with month and sales columns and join it instead of unpivoting

D Use UNPIVOT operator directly on all 12 columns in one query

💡 Hint

Consider SQL Server's CROSS APPLY with VALUES for better performance than UNPIVOT.

CROSS APPLY with VALUES is often faster and more flexible than UNPIVOT for large wide tables.

Practice

1. What is the main purpose of the PIVOT operation in SQL?

easy

A. To convert rows into columns for easier data summarization

B. To convert columns into rows to list data

C. To delete duplicate rows from a table

D. To join two tables based on a common column

Solution

1. Step 1: Understand what pivot does

   PIVOT changes the orientation of data by turning row values into columns.

2. Step 2: Compare with other options

   Only To convert rows into columns for easier data summarization correctly describes this; B describes unpivot, C and D are unrelated.

3. Final Answer:

   To convert rows into columns for easier data summarization -> Option A

4. Quick Check:

   Pivot = rows to columns [OK]

Hint: Pivot flips rows to columns; unpivot does the reverse [OK]

Common Mistakes:

• Confusing pivot with unpivot
• Thinking pivot deletes data
• Mixing pivot with join operations

2. Which of the following is the correct syntax to unpivot columns Q1, Q2, Q3 into rows in SQL Server?

easy

A. SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales IN (Q1, Q2, Q3)) AS Unpvt

B. SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales FOR Quarter IN ([Q1], [Q2], [Q3])) AS Unpvt

C. SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Quarter FOR Sales IN (Q1, Q2, Q3)) AS Unpvt

D. SELECT Product, Quarter, Sales FROM SalesData PIVOT (Sales FOR Quarter IN (Q1, Q2, Q3)) AS Pvt

Solution

1. Step 1: Recall UNPIVOT syntax

   UNPIVOT requires specifying the value column and the column that holds the original column names (FOR clause).

2. Step 2: Check each option

   SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales FOR Quarter IN ([Q1], [Q2], [Q3])) AS Unpvt correctly uses UNPIVOT (Sales FOR Quarter IN ([Q1], [Q2], [Q3])). Others misuse PIVOT or omit FOR clause.

3. Final Answer:

   SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales FOR Quarter IN ([Q1], [Q2], [Q3])) AS Unpvt -> Option B

4. Quick Check:

   UNPIVOT syntax = SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales FOR Quarter IN ([Q1], [Q2], [Q3])) AS Unpvt [OK]

Hint: UNPIVOT needs 'value FOR name IN (columns)' syntax [OK]

Common Mistakes:

• Using PIVOT instead of UNPIVOT
• Swapping value and FOR column names
• Omitting the FOR clause

3. Given the table Sales with columns Product, Year, Quarter, Amount, what will be the result of this query?

SELECT * FROM Sales PIVOT (SUM(Amount) FOR Quarter IN ([Q1], [Q2], [Q3], [Q4])) AS Pvt;

medium

A. An error because SUM cannot be used in PIVOT

B. A table with columns Product, Year, Quarter, Amount unchanged

C. A table with columns Product, Year, Q1, Q2, Q3, Q4 showing summed amounts per quarter

D. A table with columns Quarter, Product, Year, Amount

Solution

1. Step 1: Understand PIVOT with aggregation

   PIVOT with SUM aggregates Amount values grouped by Product and Year, creating columns for each Quarter.

2. Step 2: Analyze query output

   The output will have Product, Year, and one column per quarter (Q1 to Q4) with summed amounts.

3. Final Answer:

   A table with columns Product, Year, Q1, Q2, Q3, Q4 showing summed amounts per quarter -> Option C

4. Quick Check:

   PIVOT with SUM aggregates rows into columns [OK]

Hint: PIVOT with SUM groups and sums data into new columns [OK]

Common Mistakes:

• Expecting original rows unchanged
• Thinking PIVOT cannot use aggregation
• Confusing column order in output

4. You wrote this query to unpivot sales data:

SELECT Product, Quarter, Sales FROM SalesData UNPIVOT (Sales FOR Quarter IN (Q1, Q2, Q3)) AS Unpvt;

But it returns an error. What is the likely cause?

medium

A. The FOR clause should be after the IN clause

B. UNPIVOT cannot be used with more than two columns

C. The alias 'Unpvt' is missing parentheses

D. The column names in IN clause must be enclosed in square brackets

Solution

1. Step 1: Check syntax for column names in UNPIVOT

   In SQL Server, column names in the IN list must be enclosed in square brackets if they are identifiers like Q1, Q2, Q3.

2. Step 2: Identify error cause

   Missing brackets cause syntax error; other options are incorrect syntax rules.

3. Final Answer:

   The column names in IN clause must be enclosed in square brackets -> Option D

4. Quick Check:

   Use brackets for column names in UNPIVOT IN list [OK]

Hint: Always bracket column names in UNPIVOT IN list [OK]

Common Mistakes:

• Omitting brackets around column names
• Misplacing FOR clause
• Incorrect alias syntax

5. You have a table MonthlySales with columns Store, Jan, Feb, Mar representing sales per month. You want to create a report listing Store, Month, Sales rows. Which SQL approach correctly unpivots this data?

hard

A. Use UNPIVOT: SELECT Store, Month, Sales FROM MonthlySales UNPIVOT (Sales FOR Month IN ([Jan], [Feb], [Mar])) AS Unpvt;

B. Use PIVOT: SELECT Store, Month, Sales FROM MonthlySales PIVOT (SUM(Sales) FOR Month IN ([Jan], [Feb], [Mar])) AS Pvt;

C. Use JOIN to combine Jan, Feb, Mar columns into rows

D. Use GROUP BY Store, Month without unpivot

Solution

1. Step 1: Identify the goal

   We want to transform columns Jan, Feb, Mar into rows with Month and Sales columns.

2. Step 2: Choose correct SQL operation

   UNPIVOT converts columns into rows; Use UNPIVOT: SELECT Store, Month, Sales FROM MonthlySales UNPIVOT (Sales FOR Month IN ([Jan], [Feb], [Mar])) AS Unpvt; uses correct syntax for this.

3. Step 3: Evaluate other options

   PIVOT aggregates rows into columns (wrong direction), JOIN is complex and unnecessary, GROUP BY alone doesn't reshape columns.

4. Final Answer:

   Use UNPIVOT: SELECT Store, Month, Sales FROM MonthlySales UNPIVOT (Sales FOR Month IN ([Jan], [Feb], [Mar])) AS Unpvt; -> Option A

5. Quick Check:

   Unpivot columns to rows = Use UNPIVOT: SELECT Store, Month, Sales FROM MonthlySales UNPIVOT (Sales FOR Month IN ([Jan], [Feb], [Mar])) AS Unpvt; [OK]

Hint: Unpivot columns to rows using UNPIVOT with FOR and IN [OK]

Common Mistakes:

• Using PIVOT instead of UNPIVOT
• Trying to reshape with GROUP BY only
• Overcomplicating with JOINs