Pivot And Unpivot In SQL Server

PIVOT and UNPIVOT are two relational operators that are used to convert a table expression into another. PIVOT is used when we want to transfer data from row level to column level and UNPIVOT is used when we want to convert data from column level to row level. PIVOT and UNPIVOT relational operators are used to generate a multidimensional reporting. Today we will discuss both the operators. PIVOT and UNPIVOT relational operators are used to generate an interactive table that quickly combines and compares large amount of data.

Firstly, we will create a table and insert some data into the table.

Firstly, create an Employee Table

1. CREATE TABLE Employee  
2. (  
3. Name [nvarchar](max),  
4. [Year] [int] ,  
5. Sales [int]  
6. )  

Insert the following data into the table

1. INSERT INTO Employee  
2. SELECT 'Pankaj',2010,72500 UNION ALL  
3. SELECT 'Rahul',2010,60500 UNION ALL  
4. SELECT 'Sandeep',2010,52000 UNION ALL  
5. SELECT 'Pankaj',2011,45000 UNION ALL  
6. SELECT 'Sandeep',2011,82500 UNION ALL  
7. SELECT 'Rahul',2011,35600 UNION ALL  
8. SELECT 'Pankaj',2012,32500 UNION ALL  
9. SELECT 'Pankaj',2010,20500 UNION ALL  
10. SELECT 'Rahul',2011,200500 UNION ALL  
11. SELECT 'Sandeep',2010,32000   

Now we check data of Employee table.

1. SELECT * FROM Employee;  

Output



We use above Employee table for PIVOT and UNPIVOT relational operator examples. First we read about PIVOT realational operator.

PIVOT

PIVOT relational operator convert data from row level to column level. PIVOT rotates a table-valued expression by turning the unique values from one column in the expression into multiple columns in the output. Using PIVOT operator we can perform aggregate operation where we required.

Syntax

1. SELECT <non-pivoted column>,  
2.        <list of pivoted column>  
3. FROM  
4. (<SELECT query  to produces the data>)  
5.     AS <alias name>  
6. PIVOT  
7. (  
8. <aggregation function>(<column name>)  
9. FOR  
10. [<column name that  become column headers>]  
11.     IN ( [list of  pivoted columns])  
12.   
13. ) AS <alias name  for  pivot table>  

Let us take some examples.

Example 1

1. SELECT [Year], Pankaj,Rahul,Sandeep FROM   
2. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
3. PIVOT  
4. (  
5. SUM(Sales) FOR Name IN (Pankaj,Rahul,Sandeep)) AS Tab2  
6. ORDER BY [Tab2].[Year]  

Output:

In above query we calculate the sum of sales for Pankaj, Rahul and Sandeep employee corresponding to year value.

Example 2

1. SELECT Name, 2010,2011,2012 FROM   
2. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
3. PIVOT  
4. (  
5. SUM(Sales) FOR [Year] IN (2010,2011,2012)) AS Tab2  
6. ORDER BY Tab2.Name  

Output:



When we execute above query, SQL Server throws an error because we can’t provide integer value as a column name directly. To remove this error use the brackets before each integer value as in the following code snippet:

1. SELECT Name, [2010],[2011],[2012] FROM   
2. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
3. PIVOT  
4. (  
5. SUM(Sales) FOR [Year] IN ([2010],[2011],[2012])) AS Tab2  
6. ORDER BY Tab2.Name  

Output



Example 3
In previous examples we wrote the name of pivot column. This approach is useful if we know all possible values for pivot column. But it is not fix that column always remain same, pivot column may be increased or decreased in future.

Let us take previous example. In previous example we wrote 2010,2011 and 2012 as pivot column. But it is not fix that these column will not change in future , so what should we do if there is a possibility that column number may change in future.

In such a condition we should use dynamic query. Firstly, retrieve all unique values from pivot column and after that write a dynamic query to execute it with pivot query at run time.

Now we execute example 2, but using dynamic query.

1. /*Declare Variable*/  
2. DECLARE @Pivot_Column [nvarchar](max);  
3. DECLARE @Query [nvarchar](max);  
4.   
5. /*Select Pivot Column*/  
6. SELECT @Pivot_Column= COALESCE(@Pivot_Column+',','')+ QUOTENAME(Year) FROM  
7. (SELECT DISTINCT [Year] FROM Employee)Tab  
8.   
9. /*Create Dynamic Query*/  
10. SELECT @Query='SELECT Name, '+@Pivot_Column+'FROM   
11. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
12. PIVOT  
13. (  
14. SUM(Sales) FOR [Year] IN ('+@Pivot_Column+')) AS Tab2  
15. ORDER BY Tab2.Name'  
16.   
17. /*Execute Query*/  
18. EXEC  sp_executesql  @Query  

Output


UNPIVOT
UNPIVOT relational operator is reverse process of PIVOT relational operator. UNPIVOT relational operator convert data from column level to row level.

Example 4:

Suppose that output of example 2 is stored in Temp Variable. Now we want to rotate column identifiers Pankaj, Sandeep, Rahul into row values. For this we use the UNPIVOT relational operator.

Declare Temp Variable

1. DECLARE @Tab TABLE  
2. (  
3. [Year] int,  
4. Pankaj int,  
5. Rahul int,  
6. Sandeep int  
7. )  

Insert Value in Temp Variable

1. INSERT INTO @Tab  
2. SELECT [Year], Pankaj,Rahul,Sandeep FROM   
3. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
4. PIVOT  
5. (  
6. SUM(Sales) FOR Name IN (Pankaj,Rahul,Sandeep)) AS Tab2  
7. ORDER BY [Tab2].[Year]  

Perform UNPIVOT Operation

1. SELECT Name,[Year] , Sales FROM @Tab t  
2. UNPIVOT  
3. (  
4. Sales FOR Name IN (Pankaj,Rahul,Sandeep)  
5. ) AS TAb2  

Output



We can perform first PIVOT operation and after that UNPIVOT operation on same table in single query as in the following code snippet.

1. SELECT Name,[Year] , Sales FROM   
2. (  
3. SELECT [Year], Pankaj,Rahul,Sandeep FROM   
4. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
5. PIVOT  
6. (  
7. SUM(Sales) FOR Name IN (Pankaj,Rahul,Sandeep)) AS Tab2  
8. )Tab  
9. UNPIVOT  
10. (  
11. Sales FOR Name IN (Pankaj,Rahul,Sandeep)  
12. ) AS TAb2  

Output



Note
UNPIVOT operation is a reverse process of PIVOT operation, but UNPIVOT is not the exact reverse of PIVOT. If PIVOT performs an aggregation and merges multiple rows into a single row in the output, then UNPIVOT can’t reproduce the original table-valued expression result because rows have been merged. So conclusion is that if PIVOT operation merges multiple row in a single row, then UNPIVOT operation can’t retrieve original table from the output of PIVOT operation. But if PIVOT operation doesn’t merge multiple row in a single row, then UNPIVOT operation can retrieve original table from the output of PIVOT operation.

Let us take an example

Case 1(PIVOT Merger Multiple Row) :


Now we perform PIVOT and UNPIVOT operation for this table and compare the resultant table from this table.

1. SELECT Name,[Year] , Sales FROM   
2. (  
3. SELECT [Year], Pankaj,Rahul,Sandeep FROM   
4. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
5. PIVOT  
6. (  
7. SUM(Sales) FOR Name IN (Pankaj,Rahul,Sandeep)) AS Tab2  
8. )Tab  
9. UNPIVOT  
10. (  
11. Sales FOR Name IN (Pankaj,Rahul,Sandeep)  
12. ) AS TAb2  

Output



We can see that both the tables are not same. First table contain 10 rows but the above  table contains only 7 rows. This difference occur due to the PIVOT operation. PIVOT operation merge the (4,7,8) row in a single row and merge the (3,5) row in a single row. So UNPIVOT operation can’t retrieve original table.

Case 2(PIVOT doesn’t Merger Multiple Row)


Now we perform PIVOT and UNPIVOT operation for this table and compare the resultant table from this table.

1. SELECT Name,[Year] , Sales FROM   
2. (  
3. SELECT [Year], Pankaj,Rahul,Sandeep FROM   
4. (SELECT Name, [Year] , Sales FROM Employee )Tab1  
5. PIVOT  
6. (  
7. SUM(Sales) FOR Name IN (Pankaj,Rahul,Sandeep)) AS Tab2  
8. )Tab  
9. UNPIVOT  
10. (  
11. Sales FOR Name IN (Pankaj,Rahul,Sandeep)  
12. ) AS TAb2  

Output


We can see that both the tables are same.

Conclusion
Today, we read PIVOT and UNPIVOT relational operator and learned how to convert a table expression into another. We should remember that UNPIVOT operation is the reverse process of PIVOT operation, but UNPIVOT is not the exact reverse of PIVOT, it might or might not be exactly the same