SQL Server query help with pulling rows out of tangled mess of a table layout

This might be long and I apologize up front, I can get "wordy" trying to explain myself. Haven't learned to be as concise as I should be.

My issue is that I'm held back by my lack of experience and what I believe is a poorly laid out table. I'm needing to pull certain columns out of a table and report on date differences embedded in one of the elements. (See sample below).

I cannot reorganize the table as it's part of an application that my company is using on a help desk , so I'm stuck with how it's "organized".

The structure of the table is essentially this: all column types are varchar() of different lengths, the database is SQL Server 2005 on Wind开发者_如何学Pythonows Server 2003.

    tickID   | tickGen | tickAudit
    ----------------------------------
     124567   |  1      | 2011-07-22 08:00:00 joeuser Open Ticket
     124567   |  2      | 2011-07-22 08:00:00 joeuser Open Ticket Not Assigned
     124567   |  3      | 2011-07-22 08:00:00 joeuser Open Ticket Priority Urgent
     124567   |  4      | 2011-07-22 09:00:00 freduser Open Ticket Add Assignee

Management wants to know the time difference between these to "time stamps" from the tickAudit column from when "joeuser" made the ticket "not assigned" and when "freduser" added an assignee to the ticket.

In this example I came up with these queries which "sort of works" but I believe could be better:

    declare @startTime datetime
    declare @endTime datetime
    declare @mrid varchar(6)
    set @mrid = '124567'
    select @startTime = cast(substring(tickAudit,1,19) as datetime)
      from tickHistory where tickID = @tickID and tickAudit like '%Not Assigned'
    select @endTime =  cast(substring(tickAudit,1,19) as datetime) 
      from tickHistory where tickID = @tickID and tickAudit like '%Add Assignee'
    select @mrid as 'ticket', @startTime as 'startTime', 
      @endTime as 'endTime', datediff(mi,@startTime,@endTime) as 'diff'

The query however cannot handle an issue where a "ticket" has multiple entries where it was assigned and then not assigned as in this example:

    tickID   | tickGen | tickAudit
    ----------------------------------
     124568   |  1      | 2011-07-22 08:00:00 joeuser Open Ticket
     124568   |  2      | 2011-07-22 08:00:00 joeuser Open Ticket Not Assigned
     124568   |  3      | 2011-07-22 08:00:00 joeuser Open Ticket Priority Urgent
     124568   |  4      | 2011-07-22 09:00:00 freduser Open Ticket Add Assignee
     124568   |  5      | 2011-07-22 11:00:00 freduser Open Ticket Not Assigned
     124568   |  6      | 2011-07-22 12:00:00 bobuser  Open Ticket Priority Low
     124568   |  7      | 2011-07-22 12:00:00 bobuser  Open Ticket Add Assignee

Here the "freduser" has edited the ticket and now it is "not assigned" again and two hours later "bobuser" has edit it and assigned the ticket. So there are 2 time stamps that I need to report on. My query will only return the difference of tickGen #5 and tickGen #7. It ignores the difference of tickGen #2 and tickGen #4 as well.. or as I suspect it's only displaying the last of the valid matches to the query regardless how many is returned.

And this is where I get stuck. Hopefully someone can give me some suggestions or an example to account for situations in the above example.

Here is one way to do it:

with th (tickID,tickGen,tickTime,tickType)
as
(select tickID,tickGen,
    cast(substring(tickAudit,1,19) as datetime) as tickTime,
    RIGHT(tickAudit,12)
from tickHistory
where RIGHT(tickAudit,12) in ('Not Assigned','Add Assignee'))

select t1.tickID,
    t1.tickTime startTime,
    t2.tickTime endTime,
    DATEDIFF(mi,t1.tickTime,t2.tickTime) as diff
from th as t1
    inner join th as t2
        on t1.tickID = t2.tickID
            and t1.tickGen < t2.tickGen
where t1.tickType = 'Not Assigned'
    and t2.tickType = 'Add Assignee'
    and t2.tickGen = (select MIN(tickGen) from th
                where th.tickID = t1.tickID     
                    and th.tickGen > t1.tickGen)