Db2 GROUP BY

Summary: in this tutorial, you will learn how to use the Db2 GROUP BY clause to group rows into groups.

Introduction to Db2 GROUP BY clause

When you use the SELECT statement to query data, you get a result set which consists of rows. To divide these rows into groups, you use the GROUP BY clause as shown in the following query:

SELECT
    select_list
FROM
    table_name
GROUP BY
    column1, column2,...;

This statement divides rows derived from the FROM clause into groups by one or more column expression (column1, column2, …) specified in the GROUP BY clause.

When selecting groups of rows from the database, we are interested in the characteristics of the groups, not individual rows. Therefore, we often use aggregate functions in conjunction with the GROUP BY clause.

An aggregate function takes multiple rows as an input and returns a single value for these rows. Some commonly used aggregate functions are AVG(), COUNT(), MIN(), MAX() and SUM(). For example, the COUNT() function returns the number of rows for each group. The AVG() function returns the average value of all values in the group.

Here is the common query that uses the GROUP BY clause with an aggregate function:

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

FROM
    table_name
GROUP BY
    column1,
    column2,
    ...;

Notice that any column listed in the select list that is not in the aggregate function expression must be placed in the GROUP BY clause, or you will get an error.

Db2 GROUP BY clause examples

Let’s use the books and publishers tables from the sample database for the demonstration.

1) Using Db2 GROUP BY clause with COUNT(*) function

This statement uses the GROUP BY clause with the COUNT(*) function to find the number of books by publishers:

SELECT 
    publisher_id, 
    COUNT(*) book_count
FROM 
    books
GROUP BY 
    publisher_id;

In this statement:

• The GROUP BY clause divides the rows in the books table into groups by the values in the publisher_id column.
• The COUNT(*) returns the number of rows per group.

If you want to get the publisher name instead of id, you can join the books table to the publishers table as shown in the following query:

SELECT 
    p.name  publisher, 
    COUNT(*) book_count
FROM 
    books b
    INNER JOIN publishers p
        ON p.publisher_id = b.publisher_id	
GROUP BY 
    p.name
ORDER BY 
    publisher;   

Here is the output:

2) Using Db2 GROUP BY clause with AVG() function

This example uses the GROUP BY clause to find the average rating of all books for each publisher.

SELECT 
    publishers.name publisher, 
    DECIMAL(AVG(rating),5,2) avg_rating,
    COUNT(*) book_count
FROM 
    books
    INNER JOIN publishers 
        ON publishers.publisher_id = books.publisher_id	
GROUP BY 
    publishers.name
ORDER BY
    publisher;

In this example:

• The GROUP BY clause divides the books into groups by the publisher names.
• The AVG() function returns the average rating of all books for every publisher.

Notice that we keep the COUNT(*) function in the select list to view the number of books for each publisher.

3) Using Db2 GROUP BY clause with MIN() and MAX() functions

The following example finds the minimum & maximum ratings of books for each publisher using the MIN() and MAX() function with the GROUP BY clause:

SELECT 
    publishers.name publisher, 
    MIN(rating) min_rating,
    MAX(rating) max_rating
FROM 
    books
    INNER JOIN publishers 
        ON publishers.publisher_id = books.publisher_id	
GROUP BY 
    publishers.name
ORDER BY
    publisher;

4) Using Db2 GROUP BY clause with SUM() function

This example uses the GROUP BY clause with the SUM() function to find the total pages of all books for each publisher.

SELECT 
    publishers.name publisher, 
    SUM(total_pages) total_pages
FROM 
    books
    INNER JOIN publishers 
        ON publishers.publisher_id = books.publisher_id	
GROUP BY 
    publishers.name;

In this tutorial, you have learned how to use the Db2 GROUP BY clause to divide rows into groups by one or more specified columns.