datediff between sets of dates - tsql

Question

For some reason I am struggling with the logic of what I am trying to achieve here and was wondering if any help would be forthcoming.
Please find code for the data below -
/****** Object: Table [dbo].[temp_TEST] Script Date: 09/07/2021 12:01:26 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[temp_TEST](
[TransactionId] [int] NULL,
[DateFrom] [datetime] NULL,
[DateTo] [datetime] NULL,
[totalvalue] [decimal](18, 2) NULL,
[DailyAmount] [decimal](18, 2) NULL,
[EffectiveFromDate] [datetime] NULL,
[EffectiveToDate] [datetime] NULL
) ON [PRIMARY]
GO
INSERT [dbo].[temp_TEST] ([TransactionId], [DateFrom], [DateTo], [totalvalue], [DailyAmount], [EffectiveFromDate], [EffectiveToDate]) VALUES (809228, CAST(N'2021-06-14T00:00:00.000' AS DateTime), CAST(N'2021-06-27T00:00:00.000' AS DateTime), CAST(159.16 AS Decimal(18, 2)), CAST(11.37 AS Decimal(18, 2)), CAST(N'1946-01-01T00:00:00.000' AS DateTime), CAST(N'2021-06-22T00:00:00.000' AS DateTime))
INSERT [dbo].[temp_TEST] ([TransactionId], [DateFrom], [DateTo], [totalvalue], [DailyAmount], [EffectiveFromDate], [EffectiveToDate]) VALUES (809228, CAST(N'2021-06-14T00:00:00.000' AS DateTime), CAST(N'2021-06-27T00:00:00.000' AS DateTime), CAST(159.16 AS Decimal(18, 2)), CAST(11.37 AS Decimal(18, 2)), CAST(N'2021-06-23T00:00:00.000' AS DateTime), CAST(N'2021-07-09T11:59:40.833' AS DateTime))
What I am trying to do is work out the number of days that have past between a set of dates.
So what this shows is a DateFrom and DateTo field. These are the keys dates that I want to calculate between the EffectiveFromDate and EffectiveToDate.
So what I would expect is the number of days that past between the DateFrom and DateTo....during the EffectiveFromDate and EffectiveToDate, so how many days per row have past but ONLY during the DateFrom and DateTo date.
First row - has been effective since 01/01/1946, but it ended on the 22/06/21 -that is 9 days between the DateFrom and DateTo dates. That is the 14/06 to the 22/06.
Second row - has been effective since 23/06. It's still effective hence the effectivetodate being today's date...but it's been that value since the 23/06, so that will be 5 days between the DateFrom and DateTo. Including the 23/06.
Hope I've explained myself properly.

Answer 1

Please check below, it should help.
You need to use ISNULL and DATEDIFF functions
SELECT DATEDIFF(day, '2021/08/25', ISNULL('2021/08/28', getdate())) AS DateDiff;
SELECT DATEDIFF(day, '2021/08/25', ISNULL(null, GETDATE())) AS DateDiff;
SELECT DATEDIFF(day, EffectiveFromDate, ISNULL(EffectiveToDate, GETDATE())) AS DateDiff
FROM YourTable
Based on new information in question, please play around with below case when query
SELECT DATEDIFF(day,
case when EffectiveFromDate > DateFrom then EffectiveFromDate else DateFrom end,
case when EffectiveToDate < DateTo then EffectiveToDate else DateTo end) AS DateDiff
FROM temp_TEST
it actually gives you 8 and 4 days, not 9 and 5 but this is how this function works, you can add +1 to include second date in counting, see below
SELECT DATEDIFF(day,
case when EffectiveFromDate > DateFrom then EffectiveFromDate else DateFrom end,
case when EffectiveToDate < DateTo then EffectiveToDate + 1 else DateTo + 1 end) AS DateDiff
FROM temp_TEST