Building custom aggregates with FIRST_VALUE and SQL user-defined functions

26 Oct 2023

In SQL, aggregates perform calculations on a set of values and return a single result. While popular aggregates like SUM, COUNT, and AVG are often sufficient, there are cases where you may need to create custom aggregates to handle more complex calculations.

In this blog post, we’ll explore how to build custom aggregates using the FIRST_VALUE function and SQL user-defined functions (UDFs).

Introduction
Understanding FIRST_VALUE
Creating a UDF for Custom Aggregation
Using the Custom Aggregate
Conclusion

Introduction

SQL provides a powerful set of built-in functions for handling various operations. However, there are scenarios where the available built-in functions may not meet your requirements. This is where the ability to create custom aggregates becomes valuable.

Understanding FIRST_VALUE

The FIRST_VALUE function is a powerful analytical function in SQL that allows you to retrieve the first value in an ordered set. It can be essential when building custom aggregates.

Example usage of the FIRST_VALUE function:

SELECT 
    customer_id, 
    product_name, 
    FIRST_VALUE(sale_price) OVER (PARTITION BY customer_id ORDER BY purchase_date) AS first_sale_price
FROM
    sales_table;

In the above example, the FIRST_VALUE function retrieves the first sale price for each customer based on the purchase date.

Creating a UDF for Custom Aggregation

SQL User-Defined Functions (UDFs) allow you to create your own functions that can be used in SQL statements. When building custom aggregates, you can combine the power of the FIRST_VALUE function with a UDF to perform custom calculations.

Example UDF for custom aggregation:

CREATE FUNCTION calculate_total_sales(
    customerId INT,
    startDate DATE,
    endDate DATE
)
RETURNS DECIMAL(10, 2)
BEGIN
    DECLARE @totalSales DECIMAL(10, 2);
    
    SELECT
        @totalSales = SUM(first_sale_price)
    FROM
        (SELECT 
             customer_id, 
             product_name, 
             FIRST_VALUE(sale_price) OVER (PARTITION BY customer_id ORDER BY purchase_date) AS first_sale_price
         FROM
             sales_table) AS subquery
    WHERE
        customer_id = customerId
        AND purchase_date BETWEEN startDate AND endDate;
    
    RETURN @totalSales;
END;

In the above example, we create a UDF called calculate_total_sales that takes a customer ID, start date, and end date as parameters. It uses the FIRST_VALUE function to calculate the total sales for the specified customer within the given date range.

Using the Custom Aggregate

Once the UDF is created, you can use it in SQL statements like any other built-in function.

Example usage of the custom aggregate:

SELECT 
    customer_id,
    calculate_total_sales(customer_id, '2022-01-01', '2022-12-31') AS total_sales
FROM
    customers_table;

In the above example, we use the calculate_total_sales UDF to calculate the total sales for each customer in the customers_table within the specified date range.

Conclusion

By combining the power of the FIRST_VALUE function and SQL User-Defined Functions (UDFs), you can build custom aggregates to handle more complex calculations in SQL. This gives you greater flexibility and control over your data analysis and reporting tasks.

With a solid understanding of the FIRST_VALUE function and the ability to create UDFs, you can unleash the full potential of SQL to address your specific business requirements.

Table of Contents

Introduction

Understanding FIRST_VALUE

Creating a UDF for Custom Aggregation

Using the Custom Aggregate

Conclusion

References