MySQL COUNT

 MySQL COUNT

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

Introduction to the MySQL COUNT() function

The COUNT() the function is an aggregate function that returns the number of rows in a table. The COUNT() function allows you to count all rows or only rows that match a specified condition.

The COUNT() the function has three forms: COUNT(*), COUNT(expression) and COUNT(DISTINCT expression).

COUNT(*) function

The COUNT(*) the function returns the number of rows in a result set returned by a SELECT statement. The COUNT(*) returns the number of rows including duplicate, non-NULL and NULL rows.

COUNT(expression)

The COUNT(expression) returns the number of rows that do not contain NULL values as the result of the expression.

COUNT(DISTINCT expression)

The COUNT(DISTINCT expression) returns the number of distinct rows that do not contain NULL values as the result of the expression.

The return type of the COUNT() function is BIGINT. The COUNT()  the function returns 0 if there is no matching row found.

MySQL COUNT() function illustration

Setting up a sample table

First, create a table called count_demos:

CREATE TABLE count_demos (
    id INT AUTO_INCREMENT PRIMARY KEY,
    val INT
);

Second, insert some rows into the count_demos table:

INSERT INTO count_demos(val) 
VALUES(1),(1),(2),(2),(NULL),(3),(4),(NULL),(5);

Third, query data from the count_demos table:

SELECT * FROM count_demos;

MySQL COUNT(*) example

The following statement uses the COUNT(*) function to return all rows from the count_demos table:

SELECT COUNT(*) FROM count_demos;

This example uses the COUNT(*) function with a WHERE clause to specify a condition to count only rows whose value in the column  val is 2:

SELECT COUNT(*) 
FROM count_demos
WHERE val = 2;

MySQL COUNT(expression) example

If you specify the val column in the COUNT() function, the COUNT() the function will count only rows with non-NULL values in the  val column:

SELECT COUNT(val) 
FROM count_demos;

Notice that two NULL values are not included in the result.

MySQL COUNT(DISTINCT expression) example

This example uses COUNT(DISTINCT expression) to count non-NULL and distinct values in the column val:

SELECT COUNT(DISTINCT val)
FROM count_demos;

MySQL COUNT() function practical examples

We’ll use the products table from the sample database for the next examples:

A) Using MySQL COUNT(*) function with a GROUP BY example

The COUNT(*) the function is often used with a GROUP BY clause to return the number of elements in each group.

For example, this statement uses the COUNT() function with the GROUP BY clause to return the number of products in each product line:

SELECT 
    productLine, 
    COUNT(*)
FROM
    products
GROUP BY productLine;

Similarly, this example uses the COUNT(*) function to find the number of products supplied by each vendor:

SELECT 
    productVendor, 
    COUNT(*)
FROM
    products
GROUP BY productVendor
ORDER BY COUNT(*) DESC;

B) Using MySQL COUNT(*) with a HAVING clause example

To find vendors who supply at least 9 products, you use the COUNT(*) function in the HAVING clause as shown in the following query:

SELECT 
    productVendor, 
    COUNT(*)
FROM
    products
GROUP BY productVendor
HAVING COUNT(*) >= 9
ORDER BY COUNT(*) DESC;

C) MySQL COUNT IF example

You can use a control flow expression and functions e.g., IF, IFNULL, and CASE in the COUNT() function to count rows whose values match a condition.

See the following orders table from the sample database:

The following query use COUNT() with IF function to find the number of canceled, on hold, and disputed orders from the orders table:

SELECT 
    COUNT(IF(status = 'Cancelled', 1, NULL)) 'Cancelled',
    COUNT(IF(status = 'On Hold', 1, NULL)) 'On Hold',
    COUNT(IF(status = 'Disputed', 1, NULL)) 'Disputed'
FROM
    orders;

The IF() the function returns 1 if the order’s status is canceled, on hold, or disputed, otherwise, it returns NULL.

The COUNT function only counts 1, not NULL values, therefore, the query returns the number of orders based on the corresponding status.

In this tutorial, you have learned various techniques to count the number of rows in a table using the MySQL COUNT function.