Pivot and unpivot concepts in SQL

Introduction

Pivot and unpivot help you change how data is shown. Pivot turns rows into columns, and unpivot turns columns into rows. This makes data easier to read or analyze.

You want to see sales numbers for each product as columns instead of rows.

You have survey data with answers in columns and want to list them as rows.

You want to compare monthly expenses side by side in columns.

You need to prepare data for a report that requires a different layout.

You want to summarize data by turning detailed rows into a summary table.

Syntax

SQL

/* Pivot syntax (example in SQL Server) */
SELECT <columns>, [PivotColumn1], [PivotColumn2], ...
FROM
(
  SELECT <columns>, <pivot_column>, <value_column>
  FROM <table>
) AS SourceTable
PIVOT
(
  SUM(<value_column>)
  FOR <pivot_column> IN ([PivotColumn1], [PivotColumn2], ...)
) AS PivotTable;

/* Unpivot syntax (example in SQL Server) */
SELECT <columns>, UnpivotColumn, UnpivotValue
FROM
(
  SELECT <columns>, [Column1], [Column2], ...
  FROM <table>
) AS SourceTable
UNPIVOT
(
  UnpivotValue FOR UnpivotColumn IN ([Column1], [Column2], ...)
) AS UnpivotTable;

Pivot turns row values into columns to summarize data.

Unpivot turns columns into rows to normalize or list data.

Examples

This query turns product names from rows into columns showing sales per year.

SQL

/* Pivot example: Show total sales per product per year */
SELECT Year, [Apples], [Oranges], [Bananas]
FROM
(
  SELECT Year, Product, Sales
  FROM SalesData
) AS Source
PIVOT
(
  SUM(Sales)
  FOR Product IN ([Apples], [Oranges], [Bananas])
) AS PivotTable;

This query turns month columns into rows with month names and sales values.

SQL

/* Unpivot example: Convert monthly sales columns into rows */
SELECT Year, Month, Sales
FROM
(
  SELECT Year, Jan, Feb, Mar
  FROM MonthlySales
) AS Source
UNPIVOT
(
  Sales FOR Month IN ([Jan], [Feb], [Mar])
) AS UnpivotTable;

Sample Program

This creates a table with sales data, then pivots product sales into columns by year.

SQL

CREATE TABLE SalesData (Year INT, Product VARCHAR(20), Sales INT);
INSERT INTO SalesData VALUES
(2023, 'Apples', 100),
(2023, 'Oranges', 150),
(2023, 'Bananas', 120),
(2024, 'Apples', 130),
(2024, 'Oranges', 160),
(2024, 'Bananas', 140);

SELECT Year, [Apples], [Oranges], [Bananas]
FROM
(
  SELECT Year, Product, Sales
  FROM SalesData
) AS Source
PIVOT
(
  SUM(Sales)
  FOR Product IN ([Apples], [Oranges], [Bananas])
) AS PivotTable;

Important Notes

Pivot and unpivot are useful for changing data layout without changing the data itself.

Not all SQL databases support PIVOT and UNPIVOT keywords; sometimes you use CASE statements instead.

Always check your database documentation for exact syntax and support.

Summary

Pivot changes rows into columns to summarize data.

Unpivot changes columns into rows to list data.

These help make data easier to understand and use in reports.

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