Grouping_sets

GROUPING SETS is a powerful feature in Oracle SQL used in conjunction with the GROUP BY clause. It allows you to define multiple grouping sets in a single query, which is useful for producing different levels of aggregation in a result set. This can significantly reduce the number of queries required to get multiple aggregations, making your SQL queries more efficient and compact.

1. What are Grouping Sets?

GROUPING SETS is a way to define multiple groupings in one GROUP BY clause. Normally, the GROUP BY clause allows you to aggregate data by a single set of columns. However, using GROUPING SETS, you can specify several combinations of columns to group by in one query, effectively performing multiple aggregations simultaneously.

2. Syntax of Grouping Sets

SELECT column1, column2, SUM(some_column)

FROM your_table

GROUP BY GROUPING SETS (

    (column1),

    (column2),

    (column1, column2),

    ()

);

• GROUPING SETS: This is the keyword used to define multiple grouping combinations.
• The sets inside the parentheses can be one or more columns, or an empty set (()) for the grand total.

3. How It Works

Each set inside the GROUPING SETS represents a different way of grouping the data. For example:

• (column1): Groups by column1.
• (column2): Groups by column2.
• (column1, column2): Groups by both column1 and column2.
• () (empty set): Represents a grand total, i.e., no grouping.

4. Example of Using GROUPING SETS

Consider the following table sales:

product_id	region	sales_amount
101	North	100
101	South	200
102	North	150
102	South	250

A query using GROUPING SETS to aggregate the data by product, region, and both product and region would look like this:

SELECT product_id, region, SUM(sales_amount)

FROM sales

GROUP BY GROUPING SETS (

    (product_id, region),

    (product_id),

    (region),

    ()

);

5. Output of the Example Query

The query would return results like this:

product_id	region	SUM(sales_amount)
101	North	100
101	South	200
102	North	150
102	South	250
101	NULL	300
102	NULL	400
NULL	North	250
NULL	South	450
NULL	NULL	750

• The NULL values in the product_id or region columns indicate that the grouping is for the total of that dimension.
• The grand total (NULL for both columns) gives the sum of all sales.

6. Why Use Grouping Sets?

• Efficiency: Instead of running multiple queries with different groupings, GROUPING SETS allows you to run one query and get all the required results.
• Simplicity: You don't need to manually union multiple queries to get different aggregations.
• Flexibility: It provides flexibility in aggregation, such as grouping by different combinations of columns in the same query.

7. Advanced Use: Combining with GROUPING and ROLLUP

You can combine GROUPING SETS with GROUPING and ROLLUP for more advanced aggregation.

·        GROUPING: The GROUPING function is used to distinguish between actual NULL values in the data and the NULLs generated by the grouping.

Example:

SELECT product_id, region, SUM(sales_amount),

       GROUPING(product_id) AS is_product_null,

       GROUPING(region) AS is_region_null

FROM sales

GROUP BY GROUPING SETS (

    (product_id, region),

    (product_id),

    (region),

    ()

);

In this case, GROUPING(product_id) will return 1 if the grouping is based on the grand total (i.e., product_id is NULL), and 0 if product_id is part of the aggregation.

·        ROLLUP: While GROUPING SETS allows you to specify the exact groupings you need, ROLLUP provides a hierarchical summary. You can use ROLLUP for a more generic approach when you need subtotals and grand totals.

Example:

SELECT product_id, region, SUM(sales_amount)

FROM sales

GROUP BY ROLLUP (product_id, region);

This query gives subtotals for each product_id and region and a grand total.

8. Key Differences Between GROUPING SETS, CUBE, and ROLLUP

• GROUPING SETS: Allows explicit specification of the combinations of columns you want to group by.
• CUBE: Provides all possible combinations of grouping columns, including all intersections and the grand total.
• ROLLUP: Provides hierarchical grouping with subtotal levels and a grand total at the end.

9. Practical Example Use Cases for Grouping Sets

• Financial Reporting: You may want to summarize sales by product, region, and total. Instead of running separate queries for each, GROUPING SETS can do it in one query.
• Pivot Tables: If you're building reports similar to pivot tables, GROUPING SETS can help you group data in various ways and get summaries across different dimensions.
• Data Analysis: Analyzing sales across multiple categories or dimensions (e.g., region, salesperson, and product type) without needing multiple queries.

10. Conclusion

GROUPING SETS is a powerful SQL feature in Oracle that makes multi-dimensional data aggregation more efficient and flexible. By enabling multiple groupings in a single query, it reduces the need for multiple UNION operations, saving time and resources. This feature is highly useful in scenarios requiring different levels of aggregation, like financial and sales reports.