PostgreSQL GROUPING SETS

Summary: in this tutorial, you will learn about grouping sets and how to use the PostgreSQL GROUPING SETS clause to generate multiple grouping sets in a query.

Setup a sample table

Let’s get started by creating a new table called sales for the demonstration.

DROP TABLE IF EXISTS sales;

CREATE TABLE sales (
    brand VARCHAR NOT NULL,
    segment VARCHAR NOT NULL,
    quantity INT NOT NULL,
    PRIMARY KEY (brand, segment)
);

INSERT INTO sales (brand, segment, quantity)
VALUES
    ('ABC', 'Premium', 100),
    ('ABC', 'Basic', 200),
    ('XYZ', 'Premium', 100),
    ('XYZ', 'Basic', 300)
RETURNING *;

Output:

 brand | segment | quantity
-------+---------+----------
 ABC   | Premium |      100
 ABC   | Basic   |      200
 XYZ   | Premium |      100
 XYZ   | Basic   |      300
(4 rows)

The sales table stores the number of products sold by brand and segment.

Introduction to PostgreSQL GROUPING SETS

A grouping set is a set of columns by which you group using the GROUP BY clause.

A grouping set is denoted by a comma-separated list of columns placed inside parentheses:

(column1, column2, ...)

For example, the following query uses the GROUP BY clause to return the number of products sold by brand and segment. In other words, it defines a grouping set of the brand and segment which is denoted by (brand, segment)

SELECT
    brand,
    segment,
    SUM (quantity)
FROM
    sales
GROUP BY
    brand,
    segment;

Output:

 brand | segment | sum
-------+---------+-----
 XYZ   | Basic   | 300
 ABC   | Premium | 100
 ABC   | Basic   | 200
 XYZ   | Premium | 100
(4 rows)

The following query finds the number of products sold by a brand. It defines a grouping set (brand):

SELECT
    brand,
    SUM (quantity)
FROM
    sales
GROUP BY
    brand;

Output:

 brand | sum
-------+-----
 ABC   | 300
 XYZ   | 400
(2 rows)

The following query finds the number of products sold by segment. It defines a grouping set (segment):

SELECT
    segment,
    SUM (quantity)
FROM
    sales
GROUP BY
    segment;

Output:

 segment | sum
---------+-----
 Basic   | 500
 Premium | 200
(2 rows)

The following query finds the number of products sold for all brands and segments. It defines an empty grouping set which is denoted by ().

SELECT SUM (quantity) FROM sales;

Output:

 sum
-----
 700
(1 row)

Suppose you want to get all the grouping sets using a single query. To achieve this, you can use the UNION ALL to combine all the result sets of the queries above.

Because UNION ALL requires all result sets to have the same number of columns with compatible data types, you need to adjust the queries by adding NULL to the selection list of each as shown below:

SELECT
    brand,
    segment,
    SUM (quantity)
FROM
    sales
GROUP BY
    brand,
    segment

UNION ALL

SELECT
    brand,
    NULL,
    SUM (quantity)
FROM
    sales
GROUP BY
    brand

UNION ALL

SELECT
    NULL,
    segment,
    SUM (quantity)
FROM
    sales
GROUP BY
    segment

UNION ALL

SELECT
    NULL,
    NULL,
    SUM (quantity)
FROM
    sales;

Output:

 brand | segment | sum
-------+---------+-----
 XYZ   | Basic   | 300
 ABC   | Premium | 100
 ABC   | Basic   | 200
 XYZ   | Premium | 100
 ABC   | null    | 300
 XYZ   | null    | 400
 null  | Basic   | 500
 null  | Premium | 200
 null  | null    | 700
(9 rows)

This query generated a single result set with the aggregates for all grouping sets.

Even though the above query works as you expected, it has two main problems.

• First, it is quite lengthy.
• Second, it has a performance issue because PostgreSQL has to scan the sales table separately for each query.

To make it more efficient, PostgreSQL provides the GROUPING SETS clause which is the subclause of the GROUP BY clause.

The GROUPING SETS allows you to define multiple grouping sets in the same query.

The general syntax of the GROUPING SETS is as follows:

SELECT
    c1,
    c2,
    aggregate_function(c3)
FROM
    table_name
GROUP BY
    GROUPING SETS (
        (c1, c2),
        (c1),
        (c2),
        ()
);

In this syntax, we have four grouping sets (c1,c2), (c1), (c2), and ().

To apply this syntax to the above example, you can use GROUPING SETS clause instead of the UNION ALL clause like this:

SELECT
    brand,
    segment,
    SUM (quantity)
FROM
    sales
GROUP BY
    GROUPING SETS (
        (brand, segment),
        (brand),
        (segment),
        ()
    );

Output:

 brand | segment | sum
-------+---------+-----
 null  | null    | 700
 XYZ   | Basic   | 300
 ABC   | Premium | 100
 ABC   | Basic   | 200
 XYZ   | Premium | 100
 ABC   | null    | 300
 XYZ   | null    | 400
 null  | Basic   | 500
 null  | Premium | 200
(9 rows)

This query is much shorter and more readable. In addition, PostgreSQL will optimize the number of times it scans the sales table and will not scan multiple times.

Grouping function

The GROUPING() function accepts an argument which can be a column name or an expression:

GROUPING( column_name | expression)

The column_name or expression must match with the one specified in the GROUP BY clause.

The GROUPING() function returns bit 0 if the argument is a member of the current grouping set and 1 otherwise.

See the following example:

SELECT
	GROUPING(brand) grouping_brand,
	GROUPING(segment) grouping_segment,
	brand,
	segment,
	SUM (quantity)
FROM
	sales
GROUP BY
	GROUPING SETS (
		(brand),
		(segment),
		()
	)
ORDER BY
	brand,
	segment;

As shown in the screenshot, when the value in the grouping_brand is 0, the sum column shows the subtotal of the brand.

When the value in the grouping_segment is zero, the sum column shows the subtotal of the segment.

You can use the GROUPING() function in the HAVING clause to find the subtotal of each brand like this:

SELECT
	GROUPING(brand) grouping_brand,
	GROUPING(segment) grouping_segment,
	brand,
	segment,
	SUM (quantity)
FROM
	sales
GROUP BY
	GROUPING SETS (
		(brand),
		(segment),
		()
	)
HAVING GROUPING(brand) = 0
ORDER BY
	brand,
	segment;

Summary

• Use the PostgreSQL GROUPING SETS to generate multiple grouping sets.