Understanding the underlying algorithm of FIRST

Understanding the underlying algorithm of FIRST_VALUE in SQL

26 Oct 2023

In SQL, FIRST_VALUE is a window function that allows you to retrieve the first value in a group of rows based on a specified order. It is commonly used in analytical queries to find the first occurrence of a certain attribute within a partitioned dataset.

Syntax

The syntax for using FIRST_VALUE is as follows:

FIRST_VALUE(expression) OVER (PARTITION BY column ORDER BY column [ASC|DESC])

The expression represents the column or expression from which the first value will be retrieved. The PARTITION BY clause is optional and allows you to define the partitioning of the dataset. The ORDER BY clause specifies the column that will determine the order of the rows within each partition.

Algorithm

The algorithm behind FIRST_VALUE is relatively straightforward. It retrieves the value from the first row in the partitioned dataset that meets the specified order. Here’s a high-level overview of how it works:

The dataset is divided into partitions based on the PARTITION BY clause, if specified.
Within each partition, the rows are sorted based on the ORDER BY clause.
The value from the first row of each partition, according to the specified order, is returned.

It’s important to note that FIRST_VALUE does not remove or filter out any rows from the dataset. Instead, it simply returns the value from the first row within each partition.

Example Usage

Let’s consider a simple example to illustrate the usage of FIRST_VALUE. Suppose we have a table called employees with columns employee_id, name, and salary. We want to find the first and highest salary within each department.

SELECT 
    employee_id,
    name,
    salary,
    FIRST_VALUE(salary) OVER (PARTITION BY department ORDER BY salary DESC) AS first_salary
FROM
    employees;

In this example, we use FIRST_VALUE to retrieve the first salary for each department. By sorting the rows within each partition (department) in descending order based on salary, we can get the highest salary for each department as the first value.

Conclusion

Understanding the underlying algorithm of FIRST_VALUE in SQL is crucial for leveraging its power in analytical queries. By partitioning and ordering the dataset, FIRST_VALUE enables us to retrieve the first value based on specific criteria. This function is invaluable in various data analysis scenarios, allowing for deeper insights and better decision-making.

#References

#SQL #WindowFunctions