Downsample using a calendar table

Management liked the weekly report but they wanted to see every week in 2020, not just the weeks with amenity usage. We can use a calendar table to solve this problem: the calendar table includes all of the weeks, so we can join it to the dbo.DaySpaVisit table to find our answers.

Management would also like to see the first day of each calendar week, as that provides important context to report viewers.

This exercise is part of the course

Time Series Analysis in SQL Server

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Exercise instructions

Find and include the week of the calendar year.
Include the minimum value of c.Date in each group as FirstDateOfWeek. This works because we are grouping by week.
Join the Calendar table to the DaySpaVisit table based on the calendar table's date and each day spa customer's date of visit. CustomerVisitStart is a DATETIME2 which includes time, so a direct join would only include visits starting at exactly midnight.
Group by the week of calendar year.

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

SELECT
	-- Determine the week of the calendar year
	c.___,
	-- Determine the earliest DATE in this group
    -- This is NOT the DayOfWeek column
	MIN(c.___) AS FirstDateOfWeek,
	ISNULL(SUM(dsv.AmenityUseInMinutes), 0) AS AmenityUseInMinutes,
	ISNULL(MAX(dsv.CustomerID), 0) AS HighestCustomerID,
	COUNT(dsv.CustomerID) AS NumberOfAttendees
FROM dbo.Calendar c
	LEFT OUTER JOIN dbo.DaySpaVisit dsv
		-- Connect dbo.Calendar with dbo.DaySpaVisit
		-- To join on CustomerVisitStart, we need to turn 
        -- it into a DATE type
		ON c.Date = CAST(dsv.___ AS ___)
WHERE
	c.CalendarYear = 2020
GROUP BY
	-- When we use aggregation functions like SUM or COUNT,
    -- we need to GROUP BY the non-aggregated columns
	c.___
ORDER BY
	c.CalendarWeekOfYear;

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