SQL LAST_VALUE with JOIN type

29 Sep 2023

When working with SQL queries, it is common to need to retrieve the last value of a specific column for each row in a result set. This is where the “LAST_VALUE” function comes in handy. In addition, if you need to combine this functionality with a JOIN operation, you may encounter some challenges. In this blog post, we will explore how to use the LAST_VALUE function with different JOIN types in SQL.

The LAST_VALUE Function

The LAST_VALUE function is a window function in SQL that allows you to access the last value of a column within a given window frame. It enables you to perform calculations based on rows that come after the current row, making it useful for identifying the most recent value of a column.

The basic syntax of the LAST_VALUE function is as follows:

LAST_VALUE(column_name) OVER (PARTITION BY partition_column ORDER BY order_column ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)

This syntax specifies that the LAST_VALUE function should operate on the “column_name” column and order the values based on the “order_column”. The PARTITION BY clause allows you to divide the result set into partitions, whereas the ROWS BETWEEN clause defines the window frame.

Using LAST_VALUE with JOIN

To use the LAST_VALUE function in conjunction with a JOIN operation, you need to ensure that the JOIN is performed before applying the LAST_VALUE function. This is because the LAST_VALUE function operates on the result set after the JOIN has been executed.

The specific JOIN type you use will depend on your data and the requirements of your query. Here are a few common JOIN types and examples of how to incorporate the LAST_VALUE function with each:

Inner Join The INNER JOIN type returns only the matching rows between two tables. To apply the LAST_VALUE function, you can use a subquery to perform the JOIN first and then use the LAST_VALUE function on the result set.

Example:

SELECT t1.id, t1.column_name, LAST_VALUE(t1.column_name) OVER (PARTITION BY t1.id ORDER BY t2.order_column) AS last_value
FROM table1 t1
INNER JOIN table2 t2 ON t1.id = t2.id

Left Join The LEFT JOIN type returns all rows from the left table and the matching rows from the right table. To incorporate the LAST_VALUE function, you can use a subquery to perform the JOIN first, and then apply the LAST_VALUE function on the result set.

Example:

SELECT t1.id, t1.column_name, LAST_VALUE(t1.column_name) OVER (PARTITION BY t1.id ORDER BY t2.order_column) AS last_value
FROM table1 t1
LEFT JOIN table2 t2 ON t1.id = t2.id

Right Join The RIGHT JOIN type returns all rows from the right table and the matching rows from the left table. To utilize the LAST_VALUE function, you can again use a subquery to perform the JOIN first and then apply the LAST_VALUE function on the result set.

Example:

SELECT t2.id, t1.column_name, LAST_VALUE(t1.column_name) OVER (PARTITION BY t2.id ORDER BY t2.order_column) AS last_value
FROM table1 t1
RIGHT JOIN table2 t2 ON t1.id = t2.id

Remember to customize the code to match your table and column names accordingly.

Conclusion

The SQL LAST_VALUE function is a powerful tool for retrieving the last value of a column within a window frame. By combining it with various JOIN types, you can further enhance your querying capabilities. Whether you are using an INNER JOIN, LEFT JOIN, or RIGHT JOIN, understanding how to incorporate the LAST_VALUE function will enable you to extract the desired information effectively.