Date Difference for multiple dates

Question

I have a field called 'LOG_COMMENTS' in a table  named T_PRODUCTION_WORK_LOG.
In the 'LOG_COMMENTS' whenever a request is placed on hold comments are added by the application, such as 'Status changed from Open to On Hold' and
'Status changed from On Hold to Open' along with a 'LOG_DATESTAMP' field. A request can go on and off Hold multiple times, how do I detrimine the days a request is On Hold?
I know I can use the sql function DATEDIFF ( datepart , startdate , enddate ), but how do I account for the possiblity that the request was On Hold more than once? And how would I get LOG_DATESTAMP' times for 'LOG_COMMENTS'
that contain
'Status changed from Open to On Hold' and 'Status changed from On Hold to Open' ?

Answer 1

declare @Aid1426_sampleData TABLE (
[LOG_ID] [int] NULL,
[LOG_DATESTAMP] [datetime] NULL,
[LOG_UID] [int] NULL,
[LOG_COMMENTS] [nvarchar](255) NULL
INSERT @Aid1426_sampleData ([LOG_ID], [LOG_DATESTAMP], [LOG_UID], [LOG_COMMENTS]) VALUES (1530253, CAST(0x0000A2FB00A998CC AS DateTime), 317690, N'Status changed from On Hold to Open. ')
INSERT @Aid1426_sampleData ([LOG_ID], [LOG_DATESTAMP], [LOG_UID], [LOG_COMMENTS]) VALUES (1531819, CAST(0x0000A30200D25424 AS DateTime), 317690, N'Status changed from Open to On Hold. ')
INSERT @Aid1426_sampleData ([LOG_ID], [LOG_DATESTAMP], [LOG_UID], [LOG_COMMENTS]) VALUES (1511894, CAST(0x0000A29600788C28 AS DateTime), 317690, N'Status changed from Open to On Hold.')
INSERT @Aid1426_sampleData ([LOG_ID], [LOG_DATESTAMP], [LOG_UID], [LOG_COMMENTS]) VALUES (1536836, CAST(0x0000A31D00D2D188 AS DateTime), 317690, N'Status changed from On Hold to Open.')
declare @nowWithStatusColumns table (log_id int, log_datestamp datetime, log_uid int, log_comments varchar(255), new_status varchar(20), old_status varchar(20))
insert into @nowWithStatusColumns
select *,
case
when left(log_comments,20) = 'Status changed from ' then ltrim(left(right(log_comments,len(log_comments)-19),charindex('to',right(log_comments,len(log_comments)-19))-1))
end as new_status,
case
when left(log_comments,20) = 'Status changed from ' then ltrim(replace(reverse(left(reverse(log_comments),charindex(' ot',reverse(log_comments)))),'.',''))
end as old_status
from @Aid1426_sampleData
select *, datediff(hour, n2.log_datestamp, n1.log_datestamp)
from @nowWithStatusColumns n1
left outer join @nowWithStatusColumns n2
on n1.old_status = n2.new_status
and n1.log_uid = n2.log_uid
and n2.log_datestamp = (select min(log_datestamp) from @nowWithStatusColumns where log_datestamp < n1.log_datestamp and new_status = n1.old_status and log_uid = n1.log_uid)
order by n1.log_datestamp
select n1.log_uid, sum(datediff(hour, n2.log_datestamp, n1.log_datestamp)) as hours
from @nowWithStatusColumns n1
left outer join @nowWithStatusColumns n2
on n1.old_status = n2.new_status
and n1.log_uid = n2.log_uid
and n2.log_datestamp = (select min(log_datestamp) from @nowWithStatusColumns where log_datestamp < n1.log_datestamp and new_status = n1.old_status and log_uid = n1.log_uid)
where n1.new_status = 'On Hold'
group by n1.log_uid
Give this a shot.
I took the liberty of creating new/old status columns to make it easier.
Thanks Patrick, this is very close to what I need, but it is also counting the hours between when the request
comments were changed from 'Status changed from On Hold to Open' on 2014-03-27 10:17:29 -and-  
 'Status changed from Open to On Hold' on 2014-04-03 12:45:47 which is 170 hours. Anyway you could remove that?