In PostgreSQL SQL, grouping sets offer a versatile approach to aggregate data based on multiple grouping criteria within a single query. This article delves deeper into the concept of grouping sets, highlighting their significance and practical benefits.

let’s first clarify the difference between “GROUP” and “GROUP BY”:

GROUP:

• “GROUP” by itself is not a standalone SQL keyword. It’s often part of other SQL commands or functions, like “GROUP_CONCAT” or “GROUPING SETS”, where it signifies some form of aggregation or grouping of data.
• For example, “GROUP_CONCAT” in MySQL is used to concatenate values from multiple rows into a single string, with an optional “GROUP BY” clause to group the concatenated values based on some criteria.

GROUP BY:

• “GROUP BY” is a fundamental SQL clause used to group rows that have the same values into summary rows. It’s typically used in conjunction with aggregate functions (like COUNT, SUM, AVG, etc.) to perform operations on groups of rows rather than individual rows.
• When you use “GROUP BY”, you specify one or more columns by which you want to group the result set. The database engine then groups the rows that have the same values in those columns.
• “GROUP BY” is often used in combination with aggregate functions to calculate summaries or totals for each group.

Example: Let’s illustrate the difference with an example:

Suppose we have a table named “sales” with columns “product_id”, “category”, and “sales_amount”.

• If we want to group the “sales_amount” by “category”, we’d use “GROUP BY”:

SELECT category, SUM(sales_amount) 
FROM sales 
GROUP BY category;

• If we want to concatenate the product IDs for each category into a single string, we might use “GROUP_CONCAT” (MySQL-specific):

SELECT category, GROUP_CONCAT(product_id) AS product_ids
FROM sales
GROUP BY category;

“GROUP BY” is used to specify how the data should be grouped, while “GROUP” is often part of other SQL functions or commands for data manipulation or aggregation.

Grouping Sets

1. Understanding Grouping Sets:

Grouping sets extend the functionality of the GROUP BY clause by allowing users to specify multiple grouping criteria in a single query. This enables the generation of subtotal rows for various combinations of grouped data, providing a comprehensive view of aggregated results.

2. Why Use Grouping Sets?

a. Flexibility:

Grouping sets provide unparalleled flexibility in data aggregation, allowing users to define multiple levels of grouping within a single query. This flexibility is particularly beneficial when generating complex summary reports or analyzing data from different perspectives.

b. Efficiency:

Utilizing grouping sets can lead to more efficient queries compared to alternative methods such as using multiple queries or subqueries to achieve similar results. By consolidating grouping logic into a single query, grouping sets help streamline the query execution process and improve performance.

c. Simplified Query Logic:

With grouping sets, users can express complex grouping requirements in a concise and intuitive manner. This simplification of query logic enhances readability and maintainability, making it easier to understand and modify queries as business requirements evolve.

3. Practical Example:

Let’s consider a practical example to demonstrate the advantages of grouping sets in action.

Scenario:

Imagine we have a sales database containing information about sales transactions, including product categories, regions, and sales amounts. We want to generate a summary report that provides subtotal rows for sales amounts grouped by different combinations of product categories and regions.

Using Grouping Sets:

By leveraging grouping sets, we can accomplish this task efficiently within a single query. The grouping sets allow us to specify various combinations of grouping criteria, enabling the generation of subtotal rows at different levels of aggregation.

This article will provide a step-by-step exploration of grouping sets, starting from the creation of a sample table, and culminating in practical examples of their application.

Syntax of GROUPING SETS:

The GROUPING SETS clause allows for the specification of multiple grouping sets within a single GROUP BY clause. The syntax is as follows:

SELECT column1, column2, ..., aggregate_function(column_name)
FROM table_name
GROUP BY 
GROUPING SETS (
    (column1, column2, ...),
    (column1),
    (column2),
    (),
    
    ...
);

Let’s break down the syntax of the GROUPING SETS clause step by step:

1. Selecting Columns and Applying Aggregate Functions:

• The SELECT statement specifies the columns that you want to retrieve from the table.
• It can also include aggregate functions like SUM, COUNT, AVG, etc., applied to specific columns for data aggregation.

SELECT column1, column2, ..., aggregate_function(column_name)

2. Specifying the Source Table:

• After the SELECT statement, you specify the source table from which you are retrieving the data.

FROM table_name

3. Grouping the Data:

• The GROUP BY clause is used to group the rows returned by the SELECT statement based on one or more columns.
• It divides the rows into groups, each group having the same values for the specified column(s).

GROUP BY 

4. Using GROUPING SETS:

• The GROUPING SETS clause is where you specify the different grouping sets.
• It allows you to define multiple sets of columns by enclosing them in parentheses.
• Each set represents a different grouping criteria for aggregating the data.
• An empty set () represents the grand total.

GROUP BY 
GROUPING SETS (
    (column1, column2, ...),  -- Grouping set 1
    (column1),                 -- Grouping set 2
    (column2),                 -- Grouping set 3
    (),                        -- Grand total
    ...
);

This breakdown explains how each part contributes to the overall syntax of the GROUPING SETS clause in a PostgreSQL SQL query for data aggregation.

Example

1. Creating the Sample Table:

Let’s begin by creating a sample table named “employees” to hold data about employees, including their first names, departments, and salaries.

CREATE TABLE employees (
    employee_id SERIAL PRIMARY KEY,
    first_name VARCHAR NOT NULL,
    department VARCHAR NOT NULL,
    salary NUMERIC NOT NULL
);

2. Inserting Sample Data:

Now, let’s insert some sample data into the “employees” table to work with.

INSERT INTO employees (first_name, department, salary)
VALUES
    ('John', 'Finance', 60000),
    ('Jane', 'IT', 55000),
    ('Michael', 'Finance', 58000),
    ('Michael', 'IT', 65000);

3. Example Queries:

We’ll delve into the practical advantages of using grouping sets over traditional methods, using examples to illustrate the efficiency gains.

1. Traditional Approach: UNION ALL

The first example utilizes the UNION ALL approach to calculate subtotal combinations of department-wise and employee-wise salary totals. While functional, this method involves multiple queries combined with UNION ALL, resulting in increased complexity and potentially slower performance.

-- Query 1: Using UNION ALL
SELECT department, first_name, SUM(salary) 
FROM employees 
GROUP BY department, first_name
UNION ALL
SELECT department, NULL, SUM(salary) 
FROM employees 
GROUP BY department
UNION ALL
SELECT NULL, first_name, SUM(salary) 
FROM employees 
GROUP BY first_name
UNION ALL
SELECT NULL, NULL, SUM(salary) 
FROM employees;

2. Streamlined Approach: GROUPING SETS

In contrast, the second example leverages the GROUPING SETS clause to achieve the same result in a more concise and efficient manner. By specifying grouping sets, the query generates subtotal rows at different levels of aggregation within a single execution.

-- Query 2: Using GROUPING SETS
SELECT  first_name, department, SUM(salary) 
FROM employees
GROUP BY 
GROUPING SETS(
    (first_name, department),
    (department),
    (first_name),
    ()
);

Conclusion:

Grouping sets in PostgreSQL SQL provide a superior approach to data aggregation, offering efficiency gains and streamlined query development. By embracing grouping sets, users can optimize their queries, improve performance, and unlock deeper insights from their data.