SQL Server 2017 (Linux)

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Create Table BugHistory(BugID int,OldStatus nvarchar(10),NewStatus nvarchar(10),DateModified date);
Insert Into BugHistory  VALUES (12345, N'New', N'Assigned', CAST(N'2022-06-01' AS Date))
Insert Into BugHistory  VALUES (12345, N'Assigned', N'Waiting', CAST(N'2022-06-02' AS Date))
Insert Into BugHistory  VALUES (12345, N'Waiting', N'Assigned', CAST(N'2022-06-04' AS Date))
Insert Into BugHistory  VALUES (12345, N'Assigned', N'Waiting', CAST(N'2022-06-05' AS Date))
Insert Into BugHistory  VALUES (12345, N'Waiting', N'Assigned', CAST(N'2022-06-06' AS Date))
Insert Into BugHistory  VALUES (12345, N'Assigned', N'Confirmed', CAST(N'2022-06-08' AS Date))
Insert Into BugHistory  VALUES (12345, N'Confirmed', N'Coding', CAST(N'2022-06-10' AS Date))
Insert Into BugHistory  VALUES (12345, N'Coding', N'Testing', CAST(N'2022-06-14' AS Date))
Insert Into BugHistory  VALUES (12345, N'Testing', N'Closed', CAST(N'2022-06-18' AS Date))
Insert Into BugHistory  VALUES (23456, N'New', N'Assigned', CAST(N'2022-06-10' AS Date))
Insert Into BugHistory  VALUES (23456, N'Assigned', N'Waiting', CAST(N'2022-06-10' AS Date))
Insert Into BugHistory  VALUES (23456, N'Waiting', N'Assigned', CAST(N'2022-06-15' AS Date))
Insert Into BugHistory  VALUES (23456, N'Assigned', N'Waiting', CAST(N'2022-06-15' AS Date))
Insert Into BugHistory  VALUES (23456, N'Waiting', N'Assigned', CAST(N'2022-06-16' AS Date))
Insert Into BugHistory  VALUES (23456, N'Assigned', N'Waiting', CAST(N'2022-06-17' AS Date))
Insert Into BugHistory  VALUES (23456, N'Waiting', N'Assigned', CAST(N'2022-06-19' AS Date))
Insert Into BugHistory  VALUES (23456, N'Assigned', N'Confirmed', CAST(N'2022-06-21' AS Date))
Insert Into BugHistory  VALUES (23456, N'Confirmed', N'Coding', CAST(N'2022-06-30' AS Date))
Insert Into BugHistory  VALUES (23456, N'Coding', N'Testing', CAST(N'2022-07-05' AS Date))
Insert Into BugHistory  VALUES (23456, N'Testing', N'Closed', CAST(N'2022-07-10' AS Date))
20 rows affected

with cte as 
(
	select BugID,NewStatus,
	DATEDIFF(Day,DateModified, 
	LEAD(DateModified) Over (Partition By BugID Order By DateModified)) As Df 
	from BugHistory
)
select BugID, Sum(Df) As TotalDays, 
Sum(Case When NewStatus='Waiting' Then Df Else 0 End) as TotalWaiting
from cte 
Group By BugID


BugID
TotalDays
TotalWaiting


12345
17
3

23456
30
8


Warning: Null value is eliminated by an aggregate or other SET operation.