How do you transpose rows to columns in SQL?

Use the PIVOT operator or CASE WHEN statements with aggregation functions to convert row values into column headers in SQL.

What databases support PIVOT operations?

SQL Server and Oracle support native PIVOT syntax, while MySQL and PostgreSQL require CASE WHEN logic for row-to-column transformation.

Transpose Rows to Columns in SQL | FullStackPrep.Dev

💡 Concept Name

Transposing Rows to Columns in SQL – A method to pivot data horizontally, turning values from a row group into column headers using SQL techniques like PIVOT or CASE WHEN.

📘 Quick Intro

Transposing rows into columns in SQL helps build reports where you need a cross-tabular layout. Depending on the database system, you can use PIVOT, CASE WHEN, or dynamic SQL to achieve this transformation.

🧠 Analogy / Short Story

Think of data like a guestbook: each row is a guest’s name and the event they attended. Transposing is like taking all events and laying them across the top like columns, with guest names down the side—turning attendance rows into a neat attendance matrix.

🔧 Technical Explanation

📌 Use PIVOT in SQL Server or Oracle for fixed-value transformations.
💡 Use CASE WHEN and aggregate functions in all RDBMS for manual transposition.
📈 Dynamic pivoting can be done using dynamic SQL for unknown columns.
📊 Useful in reporting, matrix views, and comparative analytics.
🛠️ Always ensure the grouped column is aggregated with SUM, MAX, COUNT, etc.

💻 Real Code Example

-- Sample table: Sales (Year, Product, Revenue)
-- Transpose Years into Columns using PIVOT (SQL Server)
SELECT *
FROM (
    SELECT Year, Product, Revenue
    FROM Sales
) AS SourceTable
PIVOT (
    SUM(Revenue)
    FOR Year IN ([2022], [2023], [2024])
) AS PivotTable;

-- Alternative using CASE WHEN
SELECT
  Product,
  SUM(CASE WHEN Year = 2022 THEN Revenue ELSE 0 END) AS Revenue_2022,
  SUM(CASE WHEN Year = 2023 THEN Revenue ELSE 0 END) AS Revenue_2023,
  SUM(CASE WHEN Year = 2024 THEN Revenue ELSE 0 END) AS Revenue_2024
FROM Sales
GROUP BY Product;

🎯 Purpose & Use Case

✅ Build matrix-style reports (e.g., revenue per year per product).
✅ Compare metrics across time or categories side-by-side.
✅ Reorganize long-form data into wide-form format for dashboards.
✅ Simplify visual representation of grouped data.

❓ Interview Q&A

Q1: What is the purpose of transposing data in SQL?
A: To reshape row-wise data into columnar format for better comparison or reporting.

Q2: Which SQL Server feature helps in row-to-column conversion?
A: The PIVOT operator.

Q3: Can all SQL databases use PIVOT?
A: No, databases like MySQL use CASE WHEN logic instead.

Q4: What happens if the column values are not known in advance?
A: You must build a dynamic SQL query to handle such cases.

Q5: Why do we use aggregate functions in transposing?

A: Because pivoting groups rows and needs to combine data (e.g., SUM or COUNT) for each column value.

Q6: How does CASE WHEN help in manual pivoting?
A: It checks for specific values and assigns them to virtual columns.

Q7: Can pivoted columns be filtered?
A: Yes, using WHERE or HAVING clauses on the final output.

Q8: What kind of data is best suited for pivoting?
A: Repetitive category or time-series data.

Q9: Can you pivot multiple columns at once?
A: It's possible but more complex—requires multiple aggregations.

Q10: Is it possible to unpivot data later?
A: Yes, using the UNPIVOT operator (in SQL Server) or restructuring CASE statements.

📝 MCQs

Q1. Which SQL clause transposes rows to columns?

GROUP BY
UNION
PIVOT
DISTINCT

Q2. What SQL construct is used in MySQL for pivoting?

JOIN
IF ELSE
CASE WHEN
UNION ALL

Q3. Which function is needed during aggregation?

MIN
SELECT
SUM or COUNT
IFNULL

Q4. How do you pivot unknown values?

PIVOT normally
Hardcode values
Use dynamic SQL
Use UNION

Q5. Which keyword comes after FOR in PIVOT?

BY
WHERE
IN
ON

Q6. What must you include in a PIVOT clause?

Subquery
HAVING
Aggregate function
ORDER BY

Q7. Which database supports PIVOT natively?

SQLite
SQL Server
MySQL 5.5
MongoDB

Q8. What’s the alternative to PIVOT in PostgreSQL?

PIVOT
JOIN
CASE WHEN
MERGE

Q9. Can pivoting be reversed?

No
Only with triggers
Yes, using UNPIVOT
Only in views

Q10. Why is pivoting used in reports?

To merge databases
To compare metrics side by side
To enforce constraints
To normalize data

💡 Bonus Insight

For better flexibility, use stored procedures with dynamic SQL to handle unknown or changing pivot column values. Always validate and test with varied datasets to ensure no loss of data integrity during transposition.

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How to Transpose Rows to Columns in SQL