SQL Server Assign Conditional Variable in Select Statement - sql-server

Question

I am not an SQL expert and need some help in figuring this out. I am writing a payroll application with SQL Server 2012 database that need to keep track of time user punched in , punched out and then calculate work time and other calculations based on work time. Work time is computed as time between 9:30 a.m. and 5:30 p.m. Here is the logic:
if the user punches in earlier than 9:30 a.m., work start time is set
to 9:30 a.m.
If the user punches in later than 9:30 a.m. work start time would be
actual time punched in.
if the user punches out after 5:30 p.m. work end time would be set at
5:30 p.m.
if the user punches out before 5:30 p.m. work end time would be set
to actual punch out time
Here the partial SQL I tried but not getting anywhere. Any help would be appreciated:
declare #start time;
declare #end time;
select #start=cast('09:30:00.0000000' as time)
select #end=cast('17:30:00.0000000' as time)
SELECT Datename(dw,LastUpdate) as WeekDay
,FORMAT(PunchIn,'MM/dd/yyyy hh:mm tt') as PunchIn
,FORMAT(PunchOut,'MM/dd/yyyy hh:mm tt') as PunchOut
,case
--employee punched in before 9:30 so take 9:30 as start time
when datediff(mi,#start,cast(PunchIn as time))<0 then (select #start='09:30:00.0000000') end
from TimeTracker
This is not the full SQL but you can see the logic I am trying to employ. My question is how do I set the start time or end time based on value in another column such as punch in time or punch out time. SQL gives an error in the select statement as part of the when clause. I can do this easily in other languages but getting stumped in SQL.

Answer 1

You were almost there!
Below is your modified SQL code. See https://learn.microsoft.com/en-us/sql/t-sql/language-elements/case-transact-sql for the full syntax of CASE stmt.
declare #start time;
declare #end time;
select #start=cast('09:30:00.0000000' as time)
select #end=cast('17:30:00.0000000' as time)
SELECT Datename(dw,LastUpdate) as WeekDay
, FORMAT(PunchIn,'MM/dd/yyyy hh:mm tt') as PunchIn
, FORMAT(PunchOut,'MM/dd/yyyy hh:mm tt') as PunchOut
, Start = case
--employee punched in before 9:30 so take 9:30 as start time
when datediff(mi,#start,cast(PunchIn as time)) < 0 then #start
else cast(PunchIn as time)
end
, Stop = case
--employee punched out after 17:30 so take 17:30 as end time
when datediff(mi,#end,cast(PunchOut as time)) > 0 then #end
else cast(PunchOut as time)
end
from TimeTracker
NOTE: Your original code contained a reference to variable #start, which is not permitted. "A SELECT statement that assigns a value to a variable must not be combined with data-retrieval operations" (actual T-SQL error message). Therefore the solution posted will display result set that consists of five columns (Weekday, PunchIn, PunchOut, Start, Stop). If you were to use variables and more than one record was returned from the query, then only the last record would be saved in these variables! To store the entire set into another table, add additional line
into [table name]
before "FROM" keyword.

Answer 2

CASE statements are used to choose specific value expressions. There must be a specific value result for the statement. CASE statements are not if conditionals... they will not evaluate to executable code.
So instead of this:
--Bad
CASE WHEN 1=0 THEN Select #x=4+#y ELSE Select #x=2+#y END
You have to do things like this:
--Better
SELECT #x = CASE WHEN 1=0 THEN 4+#y ELSE 2+#y END
This shows you can still put some code in the expression, but it must still evaluate down to a value.
For the question here, it should look like more this:
case
--employee punched in before 9:30 so take 9:30 as start time
when datediff(mi,#start,cast(PunchIn as time))<0 then '09:30:00.0000000'
else PunchIn end
Though I still question doing an assignment in here.
This code is also interesting because, in most jurisdictions I've seen, it is highly illegal to alter PunchIn times like this. If an employee clocks in before they are supposed to that can be a disciplinary issue (management will ask them not to come in early, start writing them up, eventually fire them, etc), but until that process catches up the law says you must pay them for the time they worked. Again, this is jurisdictional, so consult a lawyer, but it sure looks like stealing time from workers.