MySQL LAST_VALUE Function

The LAST_VALUE() function in MySQL is a window function that returns the last value in an ordered set of values. It is used in combination with OVER() to define the window or partition of data over which the function operates.

This function is useful for performing calculations that require access to the last row in a partition of data after sorting it according to some criteria.

Syntax

LAST_VALUE(expression) OVER (
    [PARTITION BY partition_expression]
    [ORDER BY order_expression]
    [ROWS BETWEEN window_start AND window_end]
)

expression: The column or expression whose last value you want to retrieve.
PARTITION BY: Optionally divides the result set into partitions to which the window function is applied. If not specified, the entire result set is treated as a single partition.
ORDER BY: Specifies the order in which the rows are evaluated within each partition.
ROWS BETWEEN: Defines the range of rows within the window. The default window is from the first row to the current row, but it can be adjusted.

How It Works

LAST_VALUE() looks at the ordered set of rows and returns the value from the last row of the window.
It is often used with other window functions like ROW_NUMBER(), RANK(), or LEAD() to get values over partitions of data.

Examples

1. Basic Example - Using `LAST_VALUE()`

Get the last salary value of employees sorted by their joining date:

SELECT employee_id, salary,
       LAST_VALUE(salary) OVER (ORDER BY join_date) AS last_salary
FROM employees;

This will return the last salary value for each employee when sorted by join_date.

2. Using `LAST_VALUE()` with `PARTITION BY`

Find the last salary value within each department, ordered by the joining date:

SELECT department_id, employee_id, salary,
       LAST_VALUE(salary) OVER (PARTITION BY department_id ORDER BY join_date) AS last_salary_in_department
FROM employees;

This query will return the last salary of each employee within their respective department, based on the order of their joining date.

3. Using `ROWS BETWEEN`

Get the last salary value within a window of 5 rows from the current row:

SELECT employee_id, salary,
       LAST_VALUE(salary) OVER (ORDER BY join_date ROWS BETWEEN 4 PRECEDING AND CURRENT ROW) AS last_salary_in_window
FROM employees;

This query will return the last salary for each row, looking back at a window of 5 rows.

Key Considerations

Default Window:
- By default, the window for LAST_VALUE() is from the first row to the current row. To change this, use the ROWS BETWEEN clause.
Performance:
- Window functions like LAST_VALUE() can be resource-intensive on large datasets. Proper indexing and efficient query design are important for performance.
NULL Values:
- LAST_VALUE() will return NULL if there are no values in the window or if the expression is NULL.
Order of Evaluation:
- Make sure the ORDER BY clause is carefully defined to ensure the correct row is identified as the last one in the ordered set.

Common Use Cases

Time Series Analysis:
- When analyzing time-based data, you can use LAST_VALUE() to capture the latest value for a group of records, such as the latest price or transaction in a time-series dataset.
Comparative Calculations:
- It can be used to compare the current row’s value with the last value in a partition, like comparing an employee’s salary with the most recent salary in their department.
Financial and Sales Data:
- In finance, the last value of a series (like stock price, sales figures) can be useful for reporting purposes.

Conclusion

The LAST_VALUE() function in MySQL is a powerful window function that helps retrieve the last value in an ordered set of rows. By partitioning and ordering the dataset, it provides a flexible tool for complex analyses, especially when working with time-series data or partitioned datasets. However, it should be used carefully, considering performance and window definitions, to avoid errors and inefficiencies.

Souy Soeng

MySQL LAST_VALUE Function