In SQL, the LAST_VALUE function is used to compute the last value in an ordered set of values. By default, it considers all rows in the partition when computing the last value. However, sometimes you may need to apply a WHERE clause to filter the rows before finding the last value.

Let’s suppose we have a table called sales with the following structure:

CREATE TABLE sales (
    id SERIAL PRIMARY KEY,
    product_name VARCHAR(50),
    sale_date DATE,
    sale_amount DECIMAL(10,2)
);

To find the last sale amount for a specific product, we can use the following SQL query:

SELECT product_name, sale_amount
FROM (
    SELECT product_name, sale_amount,
        LAST_VALUE(sale_amount) OVER (
            PARTITION BY product_name
            ORDER BY sale_date
            ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
        ) AS last_sale_amount
    FROM sales
    WHERE product_name = 'Example Product'
) AS subquery
WHERE sale_amount = last_sale_amount;

In this query, we use a subquery to calculate the last sale amount per product. The PARTITION BY clause ensures that the calculation is done separately for each product name. The ORDER BY clause orders the sales by date, with the earliest date at the beginning. Finally, the ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING clause makes sure that all rows in the partition are considered.

The outer query filters the result to only include rows where the sale amount matches the last sale amount for the product. By adding a WHERE clause to the subquery, we ensure that only rows corresponding to the desired product are considered.

By using the LAST_VALUE function in conjunction with a WHERE clause, you can easily find the last value meeting specific conditions in your SQL queries. This can be useful when analyzing time-series data or finding the latest entries for specific categories.

#sql #lastvalue #whereclause