When working with SQL, the LAST_VALUE function can be extremely useful for retrieving the last value in a specified window partition. In combination with the IS NULL operator, you can leverage this function to filter for specific conditions based on the last value returned.

Syntax of LAST_VALUE Function

The LAST_VALUE function is typically used in conjunction with the OVER clause to specify the window partition for which the last value should be determined. The basic syntax of the LAST_VALUE function is as follows:

LAST_VALUE(column_name) OVER (
  [PARTITION BY partition_expression]
  [ORDER BY column_name [ASC | DESC]]
)

Using the LAST_VALUE Function with IS NULL Operator

To filter for records where the last value of a particular column is NULL, you can combine the LAST_VALUE function with the IS NULL operator. Here’s an example:

SELECT column_name
FROM (
  SELECT column_name,
         LAST_VALUE(column_name) OVER (PARTITION BY partition_column ORDER BY sorting_column) AS last_value
  FROM table_name
) AS subquery
WHERE last_value IS NULL;

In the above example, the LAST_VALUE function is used to retrieve the last value of column_name within each window partition defined by partition_column, and the result is assigned as last_value. The outer query then filters the records where last_value is NULL.

Example Usage

Let’s consider a scenario where we have a table named employees with columns employee_id, name, salary, and department. We want to find the department(s) where the last recorded salary is NULL. Here’s how the query would look like:

SELECT department
FROM (
  SELECT department,
         LAST_VALUE(salary) OVER (PARTITION BY department ORDER BY employee_id) AS last_salary
  FROM employees
) AS subquery
WHERE last_salary IS NULL;

In this example, we first use the LAST_VALUE function to determine the last recorded salary for each department, ordered by employee_id. The outer query then filters the records where last_salary is NULL, giving us the department(s) where the last recorded salary is missing.

Conclusion

The combination of the LAST_VALUE function with the IS NULL operator provides a convenient way to extract records where the last value in a window partition meets a certain condition. By leveraging this functionality in SQL, you can perform more specialized analysis and filtering in your queries.

#SQL #LAST_VALUE