PostgreSQL Tutorial: NTH_VALUE Function

Summary: In this tutorial, you will learn how to use the PostgreSQL NTH_VALUE() function to get a value from the nth row in a result set.

Table of Contents

Introduction to PostgreSQL NTH_VALUE() function

The NTH_VALUE() function returns a value from the nth row in an ordered partition of a result set.

Here is the syntax of the NTH_VALUE() function:

NTH_VALUE(expression, offset)
OVER (
    [PARTITION BY partition_expression]
    [ ORDER BY sort_expression [ASC | DESC]
    frame_clause ]
)

Let’s examine the syntax of the NTH_VALUE() function in detail.

expression

The expression is the target column or expression on which the NTH_VALUE() function operates.

offset

The offset is a positive integer (greater than zero) that determines the row number relative to the first row in the window against which the expression evaluates.

PARTITION BY partition_expression

The PARTITION BY clause distributes rows of the result set into partitions to which the NTH_VALUE() function applies.

ORDER BY sort_expression

The ORDER BY clause sorts rows in each partition to which the function is applied.

frame clause

The frame_clause defines the subset (or the frame) of the current partition.

PostgreSQL NTH_VALUE() function examples

We will use the products table created in the window functions tutorial for the demonstration.

1) Using PostgreSQL NTH_VALUE() function over the result set example

This example uses the NTH_VALUE() function to return all products together with the second most expensive product:

SELECT 
    product_id,
    product_name,
    price,
    NTH_VALUE(product_name, 2) 
    OVER(
        ORDER BY price DESC
        RANGE BETWEEN 
            UNBOUNDED PRECEDING AND 
            UNBOUNDED FOLLOWING
    )
FROM 
    products;

Here is the output:

In this example:

• The ORDER BY clause sorted all products by prices from high to low
• The frame clause defined the frame start at the beginning row and end at the ending row of the result set.
• The NTH_VALUE() function return value in the product_name column of the second row of the result set after sorting and framing.

2) Using PostgreSQL NTH_VALUE() function over a partition example

This example uses the NTH_VALUE() function to return all products with the second most expensive product for each product group:

SELECT 
    product_id,
    product_name,
    price,
    group_id,
    NTH_VALUE(product_name, 2) 
    OVER(
        PARTITION BY group_id
        ORDER BY price DESC
        RANGE BETWEEN 
            UNBOUNDED PRECEDING AND 
            UNBOUNDED FOLLOWING
    )
FROM 
    products;

The following picture illustrates the output:

In this example,

• The PARTITION BY clause to distributed products into product groups (or partitions) specified by the values in the group_id column.
• The ORDER BY clause sorted the products in each product group from high to low.
• The frame clause defined the whole partition as a frame.
• And the NTH_VALUE() function returns the product name of the 2nd row of each product group.

Now, you have known how to use the PostgreSQL NTH_VALUE() function to get a value from the nth row of a result set.

See more

PostgreSQL Tutorial: Window Functions

PostgreSQL Documentation: Window Functions