SQL Server Select one from multiple rows based on calculation - sql-server-2008-r2

Question

I'm trying to find the greatest number of days (and the reviewer's badge number) it took one of several reviewers to approve a particular document in a workflow. For example, I have a table that holds several workflow approval steps (submitter, manager, controller, QA), along with their badge numbers, and date they approved. The table is called "Workflow" and has those four workflow steps mentioned above as records in the table, and the main table Design that has a one-to-many relationship with Workflow.
I'm trying to determine how many days for the longest review step (number of days), and the badge number of the reviewer for that step (who is holding up the approval workflow, basically). I've been trying to set independent variables to be used later, but not sure how to also set the badge number and I'm confused. I have tried CASE, IIF, and COALESCE but am not having any luck because I don't want the first true value returned and then stop, I want it to continue to evaluate all the steps. Here is an example of my SQL:
declare #managerTime int = 0
declare #controllerTime int = 0
declare #qaTime int = 0
SET #managerTime = (SELECT DATEDIFF(day, manager.BadgeDate, submitter.BadgeDate)
from Design d
left outer join Workflow submitter on (d.DCRId = submitter.DCRId and submitter.RoleName = 'Submitter')
left outer join Workflow manager on (d.DCRId = manager.DCRId and manager.RoleName = 'System Manager')
SET #controllerTime = (SELECT DATEDIFF(day, controller.BadgeDate, manager.BadgeDate)
from Design d
left outer join Workflow manager on (d.DCRId = manager.DCRId and manager.RoleName = 'System Manager')
left outer join Workflow controller on (d.DCRId = controller.DCRId and controller.RoleName = 'DCR Controller')

Answer 1

This is how I would do it:
Create table WorkflowDefinition with flow definiton:
Source Destination Description
Submitter System Manager Submitter -> System Manager
System Manager DCR Controller System Manager -> DCR Controller
DCR Controller QA DCR Controller -> QA
Now we use this table to join workflow elements and calculate greatest number of days:
SET #MaxTime = (SELECT MAX(DATEDIFF(day, source.BadgeDate, destination.BadgeDate))
from Design d
inner join Workflow source on d.DCRId = source.DCRId
inner join WorkflowDefinition flow on source.RoleName = flow.source
inner join Workflow destination on d.DCRId = destination.DCRId
and destination.RoleName = flow.destination
)
When we have this value we can select all workflow elements which took this exact number of days to complete:
Select
destination.BadgeNumber
from Design d
inner join Workflow source on d.DCRId = source.DCRId
inner join WorkflowDefinition flow on source.RoleName = flow.source
inner join Workflow destination on d.DCRId = destination.DCRId
and destination.RoleName = flow.destination
where DATEDIFF(day, source.BadgeDate, destination.BadgeDate) = #MaxTime
If you want to know max value of days for every type of step separately then we can do something like this: Calculate max value of days per step type and put this into temp table:
SELECT
MAX(DATEDIFF(day, source.BadgeDate, destination.BadgeDate)) as maxDays,
flow.Description as StepDescription
into #tmp
from Design d
inner join Workflow source on d.DCRId = source.DCRId
inner join WorkflowDefinition flow on source.RoleName = flow.source
inner join Workflow destination on d.DCRId = destination.DCRId
and destination.RoleName = flow.destination
group by flow.Description
And now use this table to select steps matching max number of days and step description:
Select
destination.BadgeNumber
from Design d
inner join Workflow source on d.DCRId = source.DCRId
inner join WorkflowDefinition flow on source.RoleName = flow.source
inner join Workflow destination on d.DCRId = destination.DCRId
and destination.RoleName = flow.destination
inner join #tmp on DATEDIFF(day, source.BadgeDate, destination.BadgeDate) = maxDays and StepDescription = flow.Description

Answer 2

I ended up tackling this from the "many" table (Workflow), instead of the Design table. I used the following SQL which got me the results I was after. Thank you all for trying to make sense of my ramblings.
select
w.DesignId,
w.RoleName,
w.BadgeNumber,
w.BadgeDate,
DATEDIFF(day,
(select x.BadgeDate from Workflow x
where x.BadgeDate is not null
and x.DesignId = w.DesignId
and x.StepOrder = w.StepOrder - 1),
(select b.BadgeDate from Workflow b
where b.BadgeDate is not null
and b.DesignId = w.DesignId
and b.StepOrder = w.StepOrder))
as StepDuration,
w.StepOrder,
TotalDuration = DATEDIFF(day,
(select y.BadgeDate from Workflow y
where y.RoleName = 'Submitter'
and y.DesignId = w.DesignId),
(select v.BadgeDate from Workflow v
where v.RoleName = 'Approver'
and v.DesignId = w.DesignId)),
d.VersionNumber,
d.Title
from Workflow w
inner join Design d on d.DesignId = w.DesignId