How to show NULL instead of 0 when using sum aggregate / pivoting - sql

Question

Consider this 24-hour summary. The problem is that I'm getting 0s returned for hours where values haven't been inserted yet. I believe this is due to the SUM() function call.
SELECT
section,
[21], [22], [23], [0], [1], [2], [3], [4], [5], [6], [7], [8],
[21] + [22] + [23] + [0] + [1] + [2] + [3] + [4] + [5] + [6] + [7] + [8] as s1_total
-- shift 2 fields are ommitted for brievity
FROM (
SELECT
section,
-- hours from 21:00 (1st shift) to 20:00 (2nd shift)
SUM(CASE WHEN prTime = '21:00:00' THEN Amount ELSE 0 END) AS [21],
SUM(CASE WHEN prTime = '22:00:00' THEN Amount ELSE 0 END) AS [22],
SUM(CASE WHEN prTime = '23:00:00' THEN Amount ELSE 0 END) AS [23],
SUM(CASE WHEN prTime = '00:00:00' THEN Amount ELSE 0 END) AS [0],
SUM(CASE WHEN prTime = '01:00:00' THEN Amount ELSE 0 END) AS [1],
SUM(CASE WHEN prTime = '02:00:00' THEN Amount ELSE 0 END) AS [2],
-- ... similar cases are omitted for brieviety
SUM(CASE WHEN prTime = '20:00:00' THEN Amount ELSE 0 END) AS [20]
FROM (
SELECT prTime, prDate, section01 AS Amount, 'section 1' as [Section] FROM SectionsHourlyValues
UNION
SELECT prTime, prDate, section02 AS Amount, 'section 2' as [Section] FROM SectionsHourlyValues
UNION
SELECT prTime, prDate, section03 AS Amount, 'section 3' as [Section] FROM SectionsHourlyValues
) AS U
WHERE
(prDate = CONVERT(DATE, DATEADD(HOUR, -4, CONVERT(DATETIME2(7), #dt, 104))) and prTime > '20:00:00') or
(prDate = CONVERT(DATE, #dt, 104) and prTime <= '20:00:00')
GROUP BY section
) t;
For example, running the query
DECLARE #dt varchar(10) = 'certain_date';
SELECT * from [dbo].[SectionsHourlyValues] WHERE
(prDate = CONVERT(DATE, DATEADD(HOUR, -4, CONVERT(DATETIME2(7), #dt, 104))) and prTime > '20:00:00') or
(prDate = CONVERT(DATE, #dt, 104) and prTime <= '20:00:00');
wouldn't return us the data for say 09:00:00 / section1 whereas the summary would show us 0.
Then I want to show NULL (for not yet inserted records) instead of 0. How do I do that?

Answer 1

How about replacing the 0 with NULL in your conditional aggregation ?
SUM(CASE WHEN etc... ELSE NULL END)

Answer 2

Use NULLIF
ex
NULLIF(SUM(CASE WHEN prTime = '21:00:00' THEN Amount ELSE 0 END),0)
or simply do not use ELSE
SUM(CASE WHEN prTime = '21:00:00' THEN Amount END)

Answer 3

You're getting 0 values not due to the SUM function for values not yet inserted (that would be NULL and would ignored by the SUM function), but because the ELSE of your CASE statement returns 0:
SUM(CASE WHEN prTime = '21:00:00' THEN Amount ELSE 0 END) AS [21]
To solve your problem, you can use NULLIF that is rather similar to CASE:
NULLIF(MyExpressionThatCouldReturn0, 0)
That in your case would be:
NULLIF(SUM(CASE WHEN prTime = '01:00:00' THEN Amount ELSE 0 END),0)
Otherwise, as said before, do not use ELSE in the CASE statement, used as parameter of your SUM:
SUM(CASE WHEN prTime = '21:00:00' THEN Amount END)