In SQL Server, the ROLLUP clause is a powerful tool that allows you to perform aggregate operations on multiple levels in a hierarchy. This can be particularly useful when you need to create reports or analyze data at different levels of granularity.
Let’s take a look at an example to understand how the ROLLUP clause works. Consider a table called tblPopulation with the following structure:
CREATE TABLE tblPopulation (
Country VARCHAR(100),
[State] VARCHAR(100),
City VARCHAR(100),
[Population (in Millions)] INT
)
We have populated this table with some sample data:
INSERT INTO tblPopulation VALUES('India', 'Delhi','East Delhi', 9)
INSERT INTO tblPopulation VALUES('India', 'Delhi','South Delhi', 8)
INSERT INTO tblPopulation VALUES('India', 'Delhi','North Delhi', 5.5)
INSERT INTO tblPopulation VALUES('India', 'Delhi','West Delhi', 7.5)
INSERT INTO tblPopulation VALUES('India', 'Karnataka','Bangalore', 9.5)
INSERT INTO tblPopulation VALUES('India', 'Karnataka','Belur', 2.5)
INSERT INTO tblPopulation VALUES('India', 'Karnataka','Manipal', 1.5)
INSERT INTO tblPopulation VALUES('India', 'Maharastra','Mumbai', 30)
INSERT INTO tblPopulation VALUES('India', 'Maharastra','Pune', 20)
INSERT INTO tblPopulation VALUES('India', 'Maharastra','Nagpur', 11)
INSERT INTO tblPopulation VALUES('India', 'Maharastra','Nashik', 6.5)
Now, let’s say we need to create a report on the population at three levels: City, State, and Country. One way to achieve this is by using the SUM function with the GROUP BY clause. However, this would require writing separate queries for each level and then combining the results. Additionally, ordering the result would require further work.
Fortunately, SQL Server provides a much simpler solution. By adding the WITH ROLLUP clause to the GROUP BY statement, we can obtain the desired results in a single query:
SELECT Country, [State], City, SUM([Population (in Millions)]) AS [Population (in Millions)]
FROM tblPopulation
GROUP BY Country, [State], City WITH ROLLUP
The WITH ROLLUP clause generates additional rows that represent the subtotals and grand totals for each level in the hierarchy. This eliminates the need for multiple queries and simplifies the process of creating the report.
By using the ROLLUP clause, we can easily obtain the population data at different levels of granularity without the need for complex queries or additional processing.
I hope this explanation has provided you with a clear understanding of the ROLLUP clause in SQL Server. Feel free to reach out if you have any further questions or need additional clarification.