Practical examples of using FIRST_VALUE with GROUP BY in SQL queries

26 Oct 2023

Database

In SQL, the FIRST_VALUE function allows you to retrieve the first value from a group of rows when using the GROUP BY clause. This function is handy when you need to find the first occurrence of a specific value within each group. In this article, we will explore some practical examples of using FIRST_VALUE with GROUP BY to enhance your SQL queries.

Example 1: Retrieve the earliest order date for each customer

Suppose you have a table called orders that contains information about customer orders, including the customer_id and order_date. You want to find the earliest order date for each customer. Here’s how you can achieve that using FIRST_VALUE with GROUP BY:

SELECT customer_id, FIRST_VALUE(order_date) OVER (PARTITION BY customer_id ORDER BY order_date ASC) AS earliest_order_date
FROM orders
GROUP BY customer_id;

In this query, we are using FIRST_VALUE with the OVER clause to specify that we want to retrieve the first value (order_date) for each group (customer_id). The PARTITION BY clause ensures that the grouping is done based on the customer_id, and the ORDER BY clause within the OVER clause determines the order in which the rows are considered to find the first value.

Example 2: Find the highest-paid employee in each department

Consider a table called employees that contains employee information, including the employee_id, department_id, and salary. You want to determine the highest-paid employee in each department. Here’s how you can use FIRST_VALUE with GROUP BY to achieve that:

SELECT department_id, FIRST_VALUE(employee_id) OVER (PARTITION BY department_id ORDER BY salary DESC) AS highest_paid_employee_id
FROM employees
GROUP BY department_id;

In this query, we are using FIRST_VALUE with the OVER clause to retrieve the first value (employee_id) for each group (department_id). The PARTITION BY clause ensures that the grouping is done based on the department_id, and the ORDER BY clause within the OVER clause determines the order in which the rows are considered to find the first value.

By using FIRST_VALUE with GROUP BY, you can easily retrieve the first value within each group in your SQL queries. This can be particularly useful when you need to find the earliest or highest value for each group. Experiment with these examples in your own database to see the power of FIRST_VALUE with GROUP BY firsthand!

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#SQL #Database