Using FIRST_VALUE in user-defined functions

20 Oct 2023

In this blog post, we will explore how to use the FIRST_VALUE function in user-defined functions (UDFs) in order to retrieve the first value in a sorted set of rows within the same group.

Introduction
Syntax
Example
Conclusion

Introduction

The FIRST_VALUE function in SQL is used to retrieve the first value in a set of rows based on a specific ordering. This function can be extremely useful when working with complex queries or when you need to retrieve only the first value from a group of rows.

In the context of UDFs, the FIRST_VALUE function can be applied to obtain the first value within a group directly within the function’s logic. This allows you to incorporate the functionality of FIRST_VALUE within your custom functions, providing more flexibility and control over the results.

Syntax

The syntax for using the FIRST_VALUE function within a UDF is similar to regular SQL syntax. Here is the general syntax:

FIRST_VALUE(column_name) OVER (PARTITION BY column_to_group ORDER BY column_to_order)

column_name: The name of the column from which you want to retrieve the first value.
column_to_group: The column used to group the rows.
column_to_order: The column used to determine the ordering of the rows within each group.

Example

Let’s consider a scenario where we want to create a UDF that calculates the average of a specific column for each group, and also returns the first value within each group. Here’s an example:

CREATE FUNCTION get_avg_and_first_value(column_to_calculate INT)
RETURNS TABLE (group_name TEXT, average_val NUMERIC, first_val INT)
AS $$
BEGIN
  RETURN QUERY
    SELECT group_name,
           AVG(column_to_calculate),
           FIRST_VALUE(column_to_calculate) OVER (PARTITION BY group_name ORDER BY column_to_calculate)
    FROM your_table
    GROUP BY group_name;
END;
$$ LANGUAGE plpgsql;

In this example, we define a UDF called get_avg_and_first_value that takes an input parameter column_to_calculate. The UDF returns a table with columns group_name, average_val, and first_val.

The FIRST_VALUE function is used within the UDF to retrieve the first value of column_to_calculate within each group, ordered by the column itself. This is achieved by using the OVER clause, partitioning by group_name, and ordering by column_to_calculate.

To use the UDF, you can simply call it in your SQL queries like any other function:

SELECT * FROM get_avg_and_first_value(3);

This query will return the average of column 3 for each group, along with the first value of column 3 within each group.

Conclusion

By using the FIRST_VALUE function within user-defined functions, you can incorporate this powerful functionality into your custom logic. This allows you to retrieve the first value within a group directly within your UDFs, providing more control and convenience in your data manipulation tasks.

Remember to use the correct syntax and input parameters when using FIRST_VALUE within your UDFs, and explore the possibilities of combining it with other SQL functions to achieve even more intricate data calculations. Happy coding!

References:

#tags: SQL, UDF

Table of Contents

Introduction

Syntax

Example

Conclusion