Pivot and unpivot concepts in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the year column from the sales table.

SQL

SELECT [1] FROM sales;

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

APIVOT

Byear

CSUM

Dsales

Attempts:

3 left

💡 Hint

Common Mistakes

Using aggregate functions like SUM instead of column names.

Using table names instead of column names.

The year column contains the year data we want to select from the sales table.

2fill in blank

medium

Complete the code to pivot the sales data by year using SUM of amount.

SQL

SELECT * FROM sales PIVOT (SUM(amount) FOR [1] IN (2019, 2020, 2021));

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

Asales

Bamount

Cyear

Dcategory

Attempts:

3 left

💡 Hint

Common Mistakes

Using the amount column instead of the year column for pivoting.

Using table names instead of column names.

The year column is used to pivot the sales data by year.

3fill in blank

hard

Fix the error in the UNPIVOT clause by completing the missing column name.

SQL

SELECT year, category, amount FROM sales UNPIVOT (amount FOR [1] IN (Q1, Q2, Q3, Q4));

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

Aquarter

Byear

Camount

Dcategory

Attempts:

3 left

💡 Hint

Common Mistakes

Using the amount column instead of the quarter column in UNPIVOT.

Using the year or category column incorrectly.

The quarter column is used to unpivot the quarterly sales data into rows.

4fill in blank

hard

Fill both blanks to create a pivot table showing total sales by category for each year.

SQL

SELECT category, [1] FROM sales PIVOT (SUM(amount) FOR [2] IN (2019, 2020, 2021));

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

A2019, 2020, 2021

Bamount

Cyear

Dcategory

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'amount' instead of the list of years for the first blank.

Using 'amount' or 'category' instead of 'year' for the second blank.

The years 2019, 2020, and 2021 are the columns created by pivoting on the year column.

5fill in blank

hard

Fill all three blanks to unpivot quarterly sales data into a long format with columns quarter and amount.

SQL

SELECT year, category, [1], [2] FROM sales UNPIVOT ([3] FOR quarter IN (Q1, Q2, Q3, Q4));

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

Aquarter

Bamount

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the quarter and amount columns.

Using the same name for both blanks incorrectly.

The unpivoted columns are quarter and amount. The FOR clause uses 'quarter' and the unpivoted value is 'amount'.

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