Part 6 - Transform rows into columns in sql server

Suggested Videos:
Part 3 - How does a recursive CTE work
Part 4 - Delete duplicate rows in sql
Part 5 - SQL query to find employees hired in last n months



This is another common sql server interview question. We will be using the following Countries table in this example.




SQL to create the table

Create Table Countries

(

     Country nvarchar(50),

     City nvarchar(50)

)

GO

Insert into Countries values ('USA','New York')

Insert into Countries values ('USA','Houston')

Insert into Countries values ('USA','Dallas')

Insert into Countries values ('India','Hyderabad')

Insert into Countries values ('India','Bangalore')

Insert into Countries values ('India','New Delhi')

Insert into Countries values ('UK','London')

Insert into Countries values ('UK','Birmingham')

Insert into Countries values ('UK','Manchester')

Here is the interview question:
Write a sql query to transpose rows to columns. The output should be as shown below.


Using PIVOT operator we can very easily transform rows to columns

Select Country, City1, City2, City3

From

(

  Select Country, City,

    'City'+

      cast(row_number() over(partition by Country order by Country)

             as varchar(10)) ColumnSequence

  from Countries

) Temp

pivot

(

  max(City)

  for ColumnSequence in (City1, City2, City3)

) Piv