COUNT() SQL FUNCTION

What is the COUNT() function?

The COUNT() function returns the number of rows that matches a criterion.

COUNT() syntax

The basic syntax of COUNT() is as follows.

SELECT COUNT(column_name)
FROM table_name;

Variations of the syntax achieve different goals.

• The COUNT(*) syntax allows us to count the number of rows in a table
• The COUNT(DISTINCT column) syntax allows us to count the number of distinct values in a column 
• The COUNT(CASE WHEN condition THEN column END) syntax allows us to count the number of values that fulfill conditions.

COUNT(), used with GROUP BY, is useful for counting the number of rows that belong to each group.

COUNT(), used with HAVING, is useful for filtering groups according to the number of rows they have.

We will illustrate these with the examples below.

COUNT() examples

Example 1: Counting the number of rows with COUNT(*)

The table called products contains all the products a company sells. The COUNT(*) clause allows us to calculate the number of rows in the table. 

SELECT
    COUNT(*) AS number_of_rows
FROM products

Example 2: Counting unique values with COUNT(DISTINCT …)

The COUNT(DISTINCT column) syntax allows us to count the number of unique values in a column.

For example, each product has an associated brand in the products table. We can count the number of unique products and brands in the table.

SELECT
    COUNT(DISTINCT product_id) AS unique_product_count,
    COUNT(DISTINCT brand_id) AS unique_brand_count
FROM products

Example 3: Count rows that match a condition using COUNT() with CASE WHEN 

The COUNT(CASE WHEN condition THEN column END) syntax allows us to calculate the number of rows that match a condition.

For example, in the products table, each product has a list_price. We can calculate how many products are “expensive” (having a list price of more than $500) or otherwise.

SELECT
    COUNT(CASE WHEN list_price >= 500 THEN product_id END) AS expensive_product_count,
   COUNT(CASE WHEN list_price < 500 THEN product_id END) AS cheap_product_count
FROM products

Example 4: Count rows in groups using COUNT() with GROUP BY

COUNT() can be used with GROUP BY to find the number of rows in each group.

For example, the product table contains bicycle models from 2016 to 2019. To find the distribution of the bicycle, we can use COUNT(product_id) and GROUP BY model_year to count the number of products from each year. 

SELECT
    model_year,
    COUNT(product_id) AS product_count
FROM products
GROUP BY model_year

Example 5: Filter for groups using COUNT() with GROUP BY and HAVING

In example 4, we see that COUNT() can be used with GROUP BY. We can use the HAVING statement to filter for groups using the number of rows in that group.

For example, to find the number of years that have less than 50 products, we can use the following syntax. 

SELECT
   model_year
FROM products
GROUP BY model_year
HAVING COUNT(product_id) < 50

Technical requirements

COUNT() is a function that is available to all versions of modern SQL. 

See also

• COALESCE() SQL Function
• FORMAT() SQL Function
• INSERT INTO SQL Function
• DATEDIFF() SQL Function

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