In data migration scenarios, it is common to extract data from one source and load it into another database. During this process, you may need to transform and manipulate the data before loading it. One such transformation is to extract the last value of a column from the source table and insert it into the destination table.

Understanding the SQL LAST_VALUE Function

The LAST_VALUE() function in SQL is a window function that allows you to retrieve the last value of a column within a defined window frame. It is particularly useful when you want to extract the last value in a series or perform calculations based on previous values.

The syntax of the LAST_VALUE() function is as follows:

LAST_VALUE(expression) OVER (
    [PARTITION BY partition_expression]
    ORDER BY sort_expression
    [ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]
)

Here, the expression parameter represents the column or expression you want to retrieve the last value of. The PARTITION BY clause enables you to divide the result set into partitions, while the ORDER BY clause specifies the logical order of the rows within each partition.

The ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW clause defines the window frame within which the LAST_VALUE() function operates. By default, it considers all preceding and current rows in the window frame.

Applying LAST_VALUE in Data Migration

To leverage the LAST_VALUE() function in a data migration scenario, you can use it within a SQL statement to extract the last value from the source table and insert it into the destination table.

Let’s consider an example where we have a source table called employees with columns employee_id and last_name, and we want to migrate the data to a destination table called employees_migration with the same columns. We want to insert the last name of the employee as it was in the source table.

Here’s how the SQL query would look like:

INSERT INTO employees_migration (employee_id, last_name)
SELECT DISTINCT employee_id, LAST_VALUE(last_name) OVER (
    PARTITION BY employee_id ORDER BY employee_id
    ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
) AS last_name
FROM employees;

In this example, the LAST_VALUE() function is used within a SELECT statement to return the last name of each employee in the source table. The DISTINCT keyword ensures that only distinct employee_id values are inserted into the destination table.

By specifying ORDER BY employee_id, we guarantee that the last value is selected based on the ascending order of employee_id. The ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW clause includes all rows up to and including the current row in the window frame.

Conclusion

The LAST_VALUE() function in SQL is a powerful tool for data migration scenarios when you need to extract the last value of a column from the source table and load it into the destination table. By understanding the syntax and how to apply it, you can efficiently and accurately transform and migrate your data. #SQL #DataMigration