PIVOT table returning all null data - sql

Question

Consider this query:
SELECT *
FROM (
SELECT
userid,
FORMAT(datecreated, 'yyyy-MM') AS purchasemonth,
COALESCE(amount + tip, 0) AS amt
FROM invoice
) AS SourceTable
which produces output like this:
And this pivot query in which I am trying to sum over each month:
SELECT
userid,
COALESCE([2016-08-01], 0) AS [2016-08-01],
COALESCE([2016-09-01], 0) AS [2016-09-01]
FROM (
SELECT
userid,
FORMAT(datecreated, 'yyyy-MM') AS purchasemonth,
COALESCE(amount + tip, 0) AS amt
FROM invoice
) AS SourceTable
PIVOT
(
SUM(amt)
FOR purchasemonth IN ([2016-08-01], [2016-09-01])
) AS PivotTable
which produces output like this:
There is no NULL data at all in the original query's output. The PIVOT query's output is nothing but null data (coalesced to 0). But I can't figure out why the PIVOT is not summing the data as I expected. I'm expecting there to be no NULL data in the PIVOT output either.
How can I fix the query to behave as expected?

Answer 1

PurchaseMonth in your Derived Table is a String without DAYs in it and you are comparing it to a Date with days in it for the values/column names:
So the main issue is this line:
FOR purchasemonth IN ([2016-08-01], [2016-09-01])
TO
FOR purchasemonth IN ([2016-08], [2016-09])
Once you change that you would need to change the COALESCE() statements too
and you should get what you want.
SELECT
userid,
COALESCE([2016-08], 0) AS [2016-08-01],
COALESCE([2016-09], 0) AS [2016-09-01]
FROM (
SELECT
userid,
FORMAT(datecreated, 'yyyy-MM') AS purchasemonth,
COALESCE(amount + tip, 0) AS amt
FROM invoice
) AS SourceTable
PIVOT
(
SUM(amt)
FOR purchasemonth IN ([2016-08], [2016-09])
) AS PivotTable
If you want the 01 to remain for days then simply change up the Derived Table definition to be a date or include the day in the format
So if you want to go this route change this line:
FORMAT(datecreated, 'yyyy-MM') AS purchasemonth,
To
DATEADD(day,1-DAY(datecreated),datecreated) AS purchasemonth,
you could also use this
FORMAT(datecreated, 'yyyy-MM-dd') AS purchasemonth,
But FORMAT has performance impacts that you have no reason to introduce if you don't need to.
SELECT
userid,
COALESCE([2016-08-01], 0) AS [2016-08-01],
COALESCE([2016-09-01], 0) AS [2016-09-01]
FROM (
SELECT
userid,
DATEADD(day,1-DAY(datecreated),datecreated) AS purchasemonth,
COALESCE(amount + tip, 0) AS amt
FROM invoice
) AS SourceTable
PIVOT
(
SUM(amt)
FOR purchasemonth IN ([2016-08-01], [2016-09-01])
) AS PivotTable