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Introduction
Total revenue for multiple periods
Calculating deltas
Creating "revenue in quarters" reports
Summary

## Instruction

First, let's see how our revenue has changed over time. To do so, we'll compare revenue values between consecutive years, months, or any other time periods. Such reports are commonly called year-to-year, quarter-to-quarter, and month-to-month reports. In SQL, we can use the EXTRACT() function we've seen in Part 1 to create such reports. Take a look:

SELECT
EXTRACT(year FROM order_date) AS revenue_year,
SUM(amount) AS total_revenue
FROM orders
GROUP BY EXTRACT(year FROM order_date)
ORDER BY EXTRACT(year FROM order_date);


And the result:

revenue_year total_revenue
2016 208083.98
2017 617085.21
2018 440623.90

As you probably remember, the EXTRACT(time_unit FROM date) function takes two parameters: a time_unit (such as year, quarter, month, etc.) and a date or time column. The function returns the specified part of a given date.

In our example, EXTRACT(year FROM order_date) returns the year when the order was placed (2016, 2017, or 2018). As you can see, we also use the EXTRACT() function to group all the orders and sum the revenue values for each year.

Note that we also added an ORDER BY clause to make sure the revenue values are shown in chronological order.

## Exercise

For each year, show the total revenue from all orders shipped to the USA. Show two columns: revenue_year and total_revenue_usa.

Order the rows by year.

### Stuck? Here's a hint!

Modify the query from the explanation. Add a WHERE clause with ship_country = 'USA'.