SQL - Operand data type datetime2 is invalid for subtract operator

Question

I want to subtract the value of the first row from the value of the second row of column _timestamp (shown below). _number is the ordering column i

Answer 1

It's indeed not feasible with the datetime2

DECLARE @t1 DATETIME2(7) = '2020-01-01 13:20:00.000'
DECLARE @t2 DATETIME2(7) = '2020-01-01 13:22:23.000'
 
SELECT  CONVERT(VARCHAR(50),@t2 - @t1, 108)

>> Operand data type datetime2 is invalid for subtract operator.

But when converted to a datetime, it gives a nice result ;-)

DECLARE @t1 DATETIME2(7) = '2020-01-01 13:20:00.000' 
DECLARE @t2 DATETIME2(7) = '2020-01-01 13:22:23.000'

SELECT  CONVERT(VARCHAR(50), CONVERT(DATETIME, @t2) - CONVERT(DATETIME, @t1), 108)

>> 00:02:23