How do I calculate total minutes between start and end times? - sql-server

Question

How do I calculate total minutes between start and end times? The Start/End times columns are nvarchar and I am declaring them as datetime. I'm not sure if that is my first step or not, I am new to SQL and to declaring.
The final goal is to take Total Minutes, subtract Lunch and Recess (both are minutes) and then multiply by 5 to get total instructional minutes for the week per school.
DECLARE #StartTime datetime, #Endtime datetime
SELECT --[School]
[GradeLevel]
,[StartTime]
,[EndTime]
,(#Endtime - #StartTime) AS 'TotalMinutes'
,[Lunch]
,[Resess]
,[Passing]
FROM [dbo].[StartEndTimes]
Current Output:
GradeLevel StartTime EndTime TotalMinutes Lunch Resess Passing
2-5 7:50 14:20 NULL 20 10 NULL
K-5 7:45 14:20 NULL 20 10 NULL
K-5 7:50 14:20 NULL 20 10 NULL

Answer 1

Maybe something like this is what you want?
select (datediff(minute, starttime, endtime) -lunch -recess) * 5 AS TotalInstruct
from YourTable
If you want to sum it up for all rows then try:
select sum((datediff(minute, starttime, endtime) -lunch -recess) * 5) AS TotalInstruct
from YourTable
If you want to get the number of hours per school you would have to include the schoolfield in the query and use it in the group byclause, and then the query becomes this:
select school, sum((datediff(minute, starttime, endtime) -lunch -recess) * 5) AS TotalInstruct
from YourTable
group by school
Sample SQL Fiddle for the above queries.

Answer 2

If all you want is to find the difference between two dates then you can use DATEDIFF function (http://msdn.microsoft.com/en-us/library/ms189794.aspx)
Example:
DECLARE #startdate datetime2
SET #startdate = '2007-05-05 12:10:09.3312722';
DECLARE #enddate datetime2 = '2007-05-04 12:10:09.3312722';
SELECT DATEDIFF(MINUTE, #enddate, #startdate);
If however your values are in string format you need to convert them prior to passing them to the DATEDIFF function.
Example:
DECLARE #starttexttime nvarchar(100)
SET #starttexttime = '7:50'
DECLARE #starttime datetime2
SET #starttime = CONVERT(datetime2, #starttexttime, 0)
DECLARE #endtexttime nvarchar(100)
SET #endtexttime = '17:50'
DECLARE #endtime datetime2
SET #endtime = CONVERT(datetime2, #endtexttime, 0)
SELECT DATEDIFF(MINUTE, #starttime, #endtime);