SQL LAST_VALUE with WHILE statement

29 Sep 2023

In SQL, the LAST_VALUE function is used to retrieve the last value in an ordered set of values. It is often used in combination with the OVER clause and window functions to perform complex calculations on multiple rows.

In some scenarios, you may want to use the LAST_VALUE function within a WHILE loop to iterate through a table’s rows and perform certain actions based on the last value. Let’s explore how this can be achieved.

Syntax

The syntax for using LAST_VALUE with the WHILE statement in SQL is as follows:

DECLARE @lastValue <data_type>;

WHILE <condition>
BEGIN
    SET @lastValue = LAST_VALUE(<expression>) OVER (ORDER BY <order_expression>);
    
    -- Perform actions using @lastValue
    
    -- Additional statements
    
END

Here’s a breakdown of the syntax:

@lastValue: A variable to store the last value.
<data_type>: The data type of the variable.
<condition>: A condition that determines when the WHILE loop should continue or exit.
<expression>: The column or expression from which you want to retrieve the last value.
<order_expression>: The column or expression used to specify the order in which the LAST_VALUE function should be evaluated.

Example

Let’s say we have a sales table with columns product_name and price, and we want to iterate through the rows using a WHILE loop to perform some actions based on the last price value. Here’s an example:

DECLARE @lastPrice DECIMAL(10, 2);

WHILE EXISTS (SELECT * FROM sales WHERE price IS NOT NULL)
BEGIN
    SET @lastPrice = LAST_VALUE(price) OVER (ORDER BY product_name);
    
    -- Perform actions with @lastPrice
    PRINT 'Last price: ' + CAST(@lastPrice AS VARCHAR);
    
    -- Additional statements
    
    -- Update or delete records
    UPDATE sales
    SET price = NULL
    WHERE price = @lastPrice;
    
    -- Exit condition for the loop
    IF NOT EXISTS (SELECT * FROM sales WHERE price IS NOT NULL)
        BREAK;
END

In this example, we declare a variable @lastPrice to store the last price value. The WHILE loop continues as long as there are rows in the sales table with a non-null price. Within the loop, we retrieve the last price using the LAST_VALUE function with the price column and order the results by product_name. We then perform actions using the last price value, in this case, printing it to the console and updating the records with that price to set it to null.

Remember to adapt the example code to your specific table structure and requirements.

Conclusion

By combining the LAST_VALUE function with the WHILE statement in SQL, you can iterate through a table’s rows and perform actions based on the last value. This technique can be useful in various scenarios where you need to process data sequentially using the last value as a reference.