When working with datasets, we often encounter scenarios where we need to find the first occurrence of a particular value. In SQL, we can use the FIRST_VALUE function to achieve this.

The FIRST_VALUE function allows us to retrieve the first value in an ordered set of values, based on a specific ordering criteria. Let’s explore how we can use FIRST_VALUE to find the first occurrence of a tax rate in a dataset.

Syntax of FIRST_VALUE

The syntax of the FIRST_VALUE function is as follows:

FIRST_VALUE (expression) OVER (
    [PARTITION BY partition_expression]
    ORDER BY sort_expression [ASC|DESC]
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
)

• expression: The value to retrieve the first occurrence of.
• PARTITION BY partition_expression (optional): Specifies how to partition the dataset into groups.
• ORDER BY sort_expression : Specifies the ordering criteria for the dataset.
• ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW: Specifies the range of rows used for the calculation.

Example: Finding the first occurrence of a tax rate

Let’s assume we have a dataset of products with their corresponding tax rates. We want to find the first occurrence of the tax rate for each product.

SELECT 
  product_id,
  tax_rate,
  FIRST_VALUE(tax_rate) OVER (PARTITION BY product_id ORDER BY created_at) AS first_tax_rate
FROM products

In this example, we are using the FIRST_VALUE function to retrieve the first tax rate for each product, ordered by the created_at column. By partitioning the dataset based on product_id, we ensure that the calculation is done per product.

The output will include the product_id, tax_rate, and first_tax_rate columns. The first_tax_rate column will contain the first occurrence of the tax rate for each product.

Conclusion

By utilizing the FIRST_VALUE function in SQL, we can easily find the first occurrence of a specific value in a dataset. This can be particularly useful when dealing with ordered datasets and wanting to analyze or manipulate the first unique value.