When writing SQL queries, you may encounter situations where you need to retrieve the first or last value in a specific column within a group of records. In such cases, you have two functions at your disposal: FIRST_VALUE and LAST_VALUE.
Both FIRST_VALUE and LAST_VALUE are window functions that allow you to fetch the earliest or latest value within a group, based on a specific ordering. However, they differ in how they determine this value.
FIRST_VALUE
The FIRST_VALUE function returns the value from the first row within a group, according to the specified order. It is commonly used to retrieve the initial state or starting point of a particular attribute.
Here’s an example usage of FIRST_VALUE:
SELECT employee_id, salary,
FIRST_VALUE(salary) OVER (PARTITION BY department_id ORDER BY hire_date) AS starting_salary
FROM employees;
In this example, FIRST_VALUE is used to find the starting salary for each employee within their respective department, based on the hire date.
LAST_VALUE
On the other hand, the LAST_VALUE function returns the value from the last row within a group, according to the specified order. It is often used to obtain the final state or the latest value of a certain attribute.
Let’s take a look at an example of LAST_VALUE:
SELECT employee_id, salary,
LAST_VALUE(salary) OVER (PARTITION BY department_id ORDER BY hire_date) AS latest_salary
FROM employees;
In this query, LAST_VALUE is used to find the latest salary for each employee within their department, based on the hire date.
Key Differences between FIRST_VALUE and LAST_VALUE
The primary difference between FIRST_VALUE and LAST_VALUE lies in the direction in which they fetch the value within a group.
FIRST_VALUE retrieves the earliest value within a group, whereas LAST_VALUE fetches the latest value.
Additionally, they also differ in their behavior when encountering multiple rows with the same value in the specified ordering. FIRST_VALUE will consistently return the same value for all rows with equal ordering values, whereas LAST_VALUE will change its value for each occurrence.
Conclusion
In summary, both FIRST_VALUE and LAST_VALUE are useful window functions in SQL that allow you to fetch the earliest or latest value from a group of records based on a specific ordering. Understanding their differences is key to leveraging them effectively in your queries.
References
#hashtags: SQL, window functions