PostgreSQL Tutorial: DENSE_RANK Function

Summary: In this tutorial, you are going to learn how to use the PostgreSQL DENSE_RANK() function to assign a rank to each row within a partition of a result set, with no gaps in ranking values.

Table of Contents

Introduction to PostgreSQL DENSE_RANK() function

The DENSE_RANK() assigns a rank to every row in each partition of a result set. Different from the RANK() function, the DENSE_RANK() function always returns consecutive rank values.

For each partition, the DENSE_RANK() function returns the same rank for the rows which have the same values

The following shows the syntax of the DENSE_RANK() function:

DENSE_RANK() OVER (
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...
)

The DENSE_RANK() function is applied to every row in each partition defined by the PARTITION BY clause, in the sort order specified by ORDER BY clause. It will reset the rank when crossing the partition boundary.

The PARITION BY clause is optional. If you skip it, the DENSE_RANK() function will treat the whole result set as a single partition.

PostgreSQL DENSE_RANK() function demo

First, create a table named dense_ranks that has one column:

CREATE TABLE dense_ranks (
	c VARCHAR(10)
);

Second, insert some rows into the dense_ranks table:

INSERT INTO dense_ranks(c)
VALUES('A'),('A'),('B'),('C'),('C'),('D'),('E');

Third, query data from the dense_ranks table:

SELECT c from dense_ranks;

Fourth, use the DENSE_RANK() function to assign a rank to each row in the result set:

SELECT
	c,
	DENSE_RANK() OVER (
		ORDER BY c
	) dense_rank_number
FROM
	dense_ranks;

Here is the output:

PostgreSQL DENSE_RANK() function examples

We will use the products table to demonstrate the DENSE_RANK() function.

1) Using PostgreSQL DENSE_RANK() function over a result set example

This statement uses the DENSE_RANK() function to rank products by list prices:

SELECT
	product_id,
	product_name,
	price,
	DENSE_RANK () OVER ( 
		ORDER BY price DESC
	) price_rank 
FROM
	products;

Here is the output:

In this example, we skipped the PARTITION BY clause, therefore, the DENSE_RANK() function treated the whole result set as a single partition.

The DENSE_RANK() function assigned a rank to each product based on the price order from high to low specified by the ORDER BY clause.

2) Using PostgreSQL DENSE_RANK() function over partitions example

The following example assigns a rank to every product in each product group:

SELECT
	product_id,
	product_name,
	group_id,
	price,
	DENSE_RANK () OVER ( 
		PARTITION BY group_id
		ORDER BY price DESC
	) price_rank 
FROM
	products;

This picture shows the output:

In this example, the PARTITION BY clause distributed the products into product groups. The ORDER BY clause sorted products in each group by their prices from high to low to which the DENSE_RANK() function is applied.

3) Using PostgreSQL DENSE_RANK() function with a CTE example

The following statement uses the DENSE_RANK() function with a CTE to return the most expensive product in each product group:

WITH cte AS(
	SELECT
		product_id,
		product_name,
		group_id,
		price,
		DENSE_RANK () OVER ( 
			PARTITION BY group_id
			ORDER BY price DESC
		) price_rank 
	FROM
		products
) 
SELECT 
	product_id, 
	product_name, 
	price
FROM 
	cte
WHERE 
	price_rank = 1;

In this tutorial, you have learned how to use the PostgreSQL DENSE_RANK() function to calculate a rank to each row within a partition of a result set, with no gaps in rank values.

See more

PostgreSQL Tutorial: Window Functions

PostgreSQL Documentation: Window Functions