PostgreSQL Tutorial: COUNT Function

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

Table of Contents

PostgreSQL COUNT function overview

The COUNT() function is an aggregate function that allows you to get the number of rows that match a specific condition of a query.

The following statement illustrates various ways of using the COUNT() function.

COUNT(*)

The COUNT(*) function returns the number of rows returned by a SELECT statement, including NULL and duplicates.

SELECT 
   COUNT(*) 
FROM 
   table_name
WHERE
   condition;

When you apply the COUNT(*) function to the entire table, PostgreSQL has to scan the whole table sequentially.

If you use the COUNT(*) function on a big table, the query will be slow. This is related to the PostgreSQL MVCC implementation. Because multiple transactions see different states of data at the same time, there is no direct way for COUNT(*) function to count across the whole table, therefore PostgreSQL must scan all rows.

COUNT(column)

Similar to the COUNT(*) function, the COUNT(column) function returns the number of rows returned by a SELECT clause. However, it does not consider NULL values in the column.

SELECT 
   COUNT(column) 
FROM 
   table_name
WHERE
   condition;

COUNT(DISTINCT column)

In this form, the COUNT(DISTINCT column) returns the number of unique non-null values in the column.

SELECT 
   COUNT(DISTINCT column) 
FROM 
   table_name
WHERE
   condition;

We often use the COUNT() function with the GROUP BY clause to return the number of items for each group. For example, we can use the COUNT() with the GROUP BY clause to return the number of films in each film category.

PostgreSQL COUNT function examples

Let’s use the payment table in the sample database for the demonstration.

PostgreSQL COUNT(*) example

The following statement uses the COUNT(*) function to return the number of transactions in the payment table:

SELECT
   COUNT(*)
FROM
   payment;

Here is the output:

PostgreSQL COUNT(DISTINCT column) example

To get the distinct amounts which customers paid, you use the COUNT(DISTINCT amount) function as shown in the following example:

SELECT
	COUNT (DISTINCT amount)
FROM
	payment;

PostgreSQL COUNT() with GROUP BY clause

To get the number of payments by the customer, you use the GROUP BY clause to group the payments into groups based on customer id, and use the COUNT() function to count the payments for each group.

The following query illustrates the idea:

SELECT
	customer_id,
	COUNT (customer_id)
FROM
	payment
GROUP BY
	customer_id;

Here is the partial output:

PostgreSQL COUNT() with HAVING clause

You can use the COUNT function in a HAVING clause to apply a specific condition to groups. For example, the following statement finds customers who have made more than 40 payments:

SELECT
	customer_id,
	COUNT (customer_id)
FROM
	payment
GROUP BY
	customer_id
HAVING
	COUNT (customer_id) > 40;

In this tutorial, you have learned how to use the PostgreSQL COUNT() function to return the number of rows in a table.

See more

PostgreSQL Tutorial: Aggregate Functions

PostgreSQL Documentation: Aggregate Functions