SQL Server 2008 CASE Logic in SELECT statement - sql-server-2008

Question

Hello again SQL Server 2008 gurus.
I need to apply the following rules to the setting of a worker's start and end times for their work day (hourly employees) in a SELECT statement. I apologize in advance for my SQL ignorance.
The rule is to set their start time to a value stored in a table field for that worker, if they login on or before their start time (a time stored in the worker starttime column) and therefore get credit for starting at their start time.
If they log out within a 10 minute period before or anytime after their end time stored in a column for the worker, they get credit for their full day, another value stored in a column of the worker table, otherwise they are penalized some percentage of an hour, i.e. their log out time rounded to .25 of an hour less closest to the time they logged out. i.e. if they are set to log out at 4:30, and they log out at 4:18, their log out time is 4:15. If they log out at 4:20, and are set to log out at 4:30, their log out time is 4:30.
The first rule applies to all hourly employees where their workday hours is less than or equal to their expected workday value. The caveat is, for those where overtime is ok (a bit value set to 1). If overtime is permitted, the number of billable hours can exceed the full day value stored for them, and therefore the value of their logout - login time can exceed their fullday value.
The question is, can these rules be calculated in the SELECT statement and if so can I get some help with the code?
The columns containing the information are:
worker.startime (TIME)
worker.endtime (TIME)
worker.overtimeallowed (BIT)
worker.workdayhours (decimal (12,2))
worker.penaltyvalue (decimal (12,2))
If it requires a UDF or stored procedure (since I'm using the Telerik ReportViewer) I'm not sure it would be supported, but that's probably another question.
So far I've gotten some help with applying some CASE logic - calculating whether a worker get's credit for their 1/2 lunch. The code that was supplied works as promised. This, I believe may be an extension to that logic - so I'll provide the code I have here:
-- for testing purposes only.
DECLARE #StartDate AS DateTime
SET #StartDate = CAST('03/25/2012' AS DATE)
DECLARE #EndDate AS DateTime
SET #EndDate = CAST('04/10/2012' AS DATE)
SELECT
w.Firstname
,w.Lastname
,wf.Login
,wf.logout
,ROUND(CAST(DATEDIFF(MI, wf.Login, wf.Logout) AS DECIMAL)/60,2)
- CASE
WHEN DATEDIFF(hour, wf.Login, wf.Logout) < w.MinimumHours THEN
w.LunchDeduction
ELSE
0
END AS [Hours Credited]
FROM Workers AS w
JOIN Workflow AS wf
ON wf.LoggedInWorkerid = w.ID
JOIN Roles AS r
ON w.RoleID = r.RoleID
WHERE (r.Descript = 'Hourly')
AND wf.Login >= #StartDate AND wf.Logout <= #EndDate
ORDER BY w.Lastname, w.Firstname

Answer 1

Here is a sample select dealing with constraints you described. CTEs create tables for testing purposes. Main query shows the calculations. You have worked with datediffs and dateadds so there is no mistery. If you haven't use % before, it is modulo operator used to round time to 15 minutes.
;with worker (ID, overtime, startTime, endTime) as
(
select 1, 1, CAST ('08:30' as time), CAST ('16:30' as time)
union all
select 2, 0, CAST ('08:30' as time), CAST ('16:30' as time)
union all
select 3, 0, CAST ('08:30' as time), CAST ('16:30' as time)
),
-- Test table of workflows
wf (workerID, login, logout) as
(
select 1, CAST ('2012-03-11 08:20' as datetime), CAST ('2012-03-11 19:33' as datetime)
union all
select 2, CAST ('2012-03-11 08:50' as datetime), CAST ('2012-03-11 16:20' as datetime)
union all
select 3, CAST ('2012-03-11 08:22' as datetime), CAST ('2012-03-11 16:18' as datetime)
)
select wf.workerID,
wf.login,
wf.logout,
-- if starttime > login return startTime else login
case when DATEDIFF(MI, w.startTime, cast (wf.login as time)) < 0
then cast(CAST (wf.login AS date) AS datetime) + w.startTime
else wf.login
end roundedLogin,
case when w.overtime = 1 -- Round to 15 minutes whenever finished
OR
-- Round to 15 minutes if left ten or more minutes before endTime
DATEDIFF(MI, cast (wf.logout as time), dateadd (MI, -10, w.endTime)) > 0
then dateadd (MI, -(DATEPART (MI, wf.logout) % 15), wf.logout)
-- stop at endTime if overtime = 0 OR left job at apropriate time
else cast(CAST (wf.logout AS date) AS datetime) + w.endTime
end roundedLogout
from worker w
inner join wf
on w.ID = wf.workerID
There will be a problem with this approach. When you start to integrate mathematics into original query you will notice that you have to write expressions evaluating roundedLogin and roundedLogout again to calculate billable hours. You cannot reuse alias defined in the same scope, but you can create derived table or view or even calculated fields. View returning columns from workflows and all additional expressions would probably be the best.
Using this view in other queries would simplify things by encapsulating logic at one place.