group by rollup

GROUP BY ROLLUP is a clause used in SQL to generate multiple levels of subtotals in the result set. It is often used in conjunction with the GROUP BY clause to produce a hierarchical breakdown of data.

When you use GROUP BY ROLLUP, the result set will include additional rows that represent different levels of aggregation. These rows will contain NULL values in the columns that represent higher-level aggregations.

For example, consider a table "Sales" with columns "Region", "Country", and "Product". If you want to get subtotals at the region, country, and product levels, you can use the GROUP BY ROLLUP clause like this:

SELECT Region, Country, Product, SUM(SalesAmount) AS TotalSales
FROM Sales
GROUP BY ROLLUP (Region, Country, Product);

The result set will include rows for each combination of region, country, and product, as well as subtotals for each level of aggregation. The rows representing higher-level aggregations will have NULL values in the corresponding columns.

Note that the number of rows returned can be significantly larger when using GROUP BY ROLLUP compared to a simple GROUP BY clause. Therefore, it's important to consider the performance implications when using this feature.