calculate_lost_customers

CREATE FUNCTION calculate_lost_customers()
RETURNS TABLE (year INT, month INT, lost_customers_count INT) AS
$$
DECLARE
    previous_date DATE;
BEGIN
    -- Get the minimum and maximum dates from the sales table
    SELECT MIN(sales_date), MAX(sales_date)
    INTO previous_date
    FROM sales;

    -- Loop through each month
    WHILE previous_date <= CURRENT_DATE - INTERVAL '6 months' LOOP
        -- Get the year and month
        year := EXTRACT(YEAR FROM previous_date);
        month := EXTRACT(MONTH FROM previous_date);

        -- Get the customers who have not made a purchase for 6 months
        SELECT COUNT(DISTINCT customer_id)
        INTO lost_customers_count
        FROM sales
        WHERE sales_date >= previous_date - INTERVAL '6 months'
        AND sales_date < previous_date;

        -- Return the result for the current month and year
        RETURN NEXT;

        -- Move to the next month
        previous_date := previous_date + INTERVAL '1 month';
    END LOOP;
END
$$ LANGUAGE plpgsql;

This code snippet shows a PostgreSQL function named calculate_lost_customers(). The function returns a table with three columns: year, month, and lost_customers_count.

The function uses a loop to iterate through each month, starting from the minimum date in the sales table up until 6 months prior to the current date. Each iteration of the loop calculates the number of lost customers for the current month by counting the distinct customer IDs from the sales table.

The function uses the INTO keyword to assign the values to the corresponding variables: previous_date for the minimum date and lost_customers_count for the customer count.

The RETURN NEXT statement returns the result for the current month and year as a row in the result set. Finally, the previous_date is incremented by 1 month to move to the next iteration.

Overall, this function calculates the number of lost customers for each month within the past 6 months based on the sales table.