Db2 COUNT

Summary: in this tutorial, you will learn how to use the DB2 COUNT() function to return the number of values in a set or the number of rows from a table.

Introduction to Db2 COUNT() function

The Db2 COUNT() function is an aggregate function that returns the number of values in a set or the number of rows in a table.

The following is the syntax of the COUNT() function:

COUNT( ALL | DISTINCT expression)

The COUNT() function accepts a set of values which can be any built-in data type except for BLOB, CLOB, DBCLOB, and XML.

The COUNT(expression) is the same as COUNT(ALL expression) which returns the number of non-null values in a set, including duplicates.

The COUNT(DISTINCT expression) returns the number of distinct non-null values.

The COUNT(*) returns the number of rows in a set, including rows that contain NULL values.

The COUNT()  returns a result of INT type. It never returns NULL.

If the number of values in a set exceeds the maximum value of the INT type, which is 2,147,483,647, you can use the COUNT_BIG() function instead.

The COUNT_BIG() behaves the same as the COUNT() function except for the type of return value that supports a larger range, i.e., BIGINT.

Db2 COUNT() function illustration

First, create a new table named count_demo that contains one integer column:

CREATE TABLE count_demo(c1 INT);

Second, insert five rows into the count_demo table:

INSERT INTO count_demo(c1) 
VALUES(1),(2),(3),(NULL),(3);

Third, view the data from the count_demo table by using the following SELECT statement:

SELECT c1 FROM count_demo;

Fourth, this statement uses the COUNT(*) function to return the number of rows from the count_demo table:

SELECT 
    COUNT(*) row_count
FROM 
    count_demo

Here is the output:

ROW_COUNT   
----------- 
5 

Fifth, this statement uses the COUNT(DISTINCT expression) to get the number of non-null rows from the count_demo table:

SELECT
    COUNT(c1) result
FROM 
    count_demo;

The output is the following:

RESULT   
----------- 
4           

Sixth, this statement uses the COUNT(DISTINCT expression) to return distinct non-null values from the count_demo table:

SELECT 
    COUNT(DISTINCT c1) result
FROM 
    count_demo;

The number of distinct non-null values in the count_demo table is as follows:

RESULT      
----------- 
3     

Db2 COUNT() examples

We’ll use the books table from the sample database to demonstrate the COUNT() function.

1) Using DB2 COUNT(*) function to return the number of rows from a table examples

This example uses the COUNT(*) function to find the number of books from the books table:

SELECT 
    COUNT(*)
FROM
    books;

Here is the output:

BOOK_COUNT  
----------- 
1091     

To get the number of books which have ISBN, you add a WHERE clause as the following:

SELECT     
    COUNT(*) book_isbn
FROM
    books
WHERE 
    isbn IS NOT NULL;

Here is the result:

BOOK_ISBN   
----------- 
984 

2) Using DB2 COUNT() function with the GROUP BY clause example

The COUNT() function is often used with the GROUP BY clause to return the number of values for each group.

This statement uses the COUNT(expression) returns the number of books with ISBN for each publisher:

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

This picture illustrates the partial output:

In this example, the GROUP BY clause groups books by publishers and the COUNT(ISBN) function returns the number of books with ISBN for every publisher.

3) Using DB2 COUNT() function with the HAVING clause example

The following statement finds the publishers that have more than 30 books, where all the books have ISBN:

SELECT 
    p.name publisher, 
    COUNT(isbn) book_with_isbn
FROM 
    books b
    INNER JOIN publishers p
        ON p.publisher_id = b.publisher_id
GROUP BY 
    p.name
HAVING 
    COUNT(isbn) > 30
ORDER BY 
    book_with_isbn DESC;	

In this tutorial, you have learned how to use the Db2 COUNT() function to get the number of values in a set or the number of rows in a table.