SQL CASE IF [VALUE] WHEN [Value] AND [VALUE] THEN, Sql Server

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create table t (
    Name varchar(32)
  , WorkOrder int
  , OnSiteStarted datetime2(0)
  , OnSiteSubmitted datetime2(0)
);
insert into t values
 ('On-Site','23022017','2017-02-23 20:31:49.000','2017-02-23 20:33:47.000')
,('Off-Site','35','2017-02-15 18:32:26.000','2017-02-15 21:11:04.000')
,('On-Site','64169086','2017-02-22 11:42:40.000','2017-02-22 15:10:17.000')
,('Off-Site','91487689','2017-02-11 13:48:11.000','2017-02-11 14:32:37.000')
,('On-Site','91494707','2017-02-13 14:39:42.000','2017-02-13 14:49:03.000')
,('Off-Site','92810031','2017-02-17 09:30:59.000','2017-02-17 09:58:19.000')
,('On-Site','92810031','2017-02-17 09:00:12.000','2017-02-17 09:30:48.000')
,('Off-Site','92810034','2017-02-17 10:01:27.000','2017-02-17 10:24:45.000')
,('On-Site','92810039','2017-03-03 12:33:41.000','2017-03-03 12:52:37.000')
,('Off-Site','92810039','2017-03-03 12:44:52.000','2017-03-03 13:41:35.000')

select 
    Name
  , WorkOrder
  , OnSiteStarted   = convert(varchar(30),OnSiteStarted,120)
  , OnSiteSubmitted = convert(varchar(30),OnSiteSubmitted,120)
  , datediff(minute
      , lead([onsitesubmitted]) over (
          partition by WorkOrder 
          order by Name 
        )
      , [onsitesubmitted]
     ) as Duration
from t
order by WorkOrder, OnSiteSubmitted

View schema
Execution time: 0 sec, rows selected: 10, rows affected: 10, absolute service time: 0,17 sec