How do you get year-to-date sales per customer?

Short answer: Filter OrderDate from Jan 1 of current year, GROUP BY CustomerId, SUM(Amount). Or use a window with PARTITION BY CustomerId and a date filter.

Sample solution

DECLARE @Start DATE = DATEFROMPARTS(YEAR(GETDATE()), 1, 1);
SELECT CustomerId, SUM(Amount) AS YtdSales
FROM Orders
WHERE OrderDate >= @Start AND OrderDate < DATEADD(DAY, 1, CAST(GETDATE() AS date))
GROUP BY CustomerId;

Half-open date ranges [start, end) avoid time-of-day bugs.