Stuff function in T-SQL select STUFF(
STUFF(RIGHT('000000' + CAST([run_duration] AS VARCHAR(6)),  6)
            , 3, 0, ':')
        , 6, 0, ':') 
    AS [LastRunDuration (HH:MM:SS)]
 from sysjobhistory


The query I have given above works fine but I am trying to understand it more clear. My observations are:


First we cast the run duration into varchar and with a length 6 and append six 0's to right of it which becomes if its is 10 then 00000010
After we use the stuff function to insert :  in between this here it is at the 3rd position so the above one becomes 00:000010
Again we use stuff  to insert the : symbol to the sixth position so it becomes 00:00:0010


But when I run the query if there is a values 10 it shows as 00:00:10.

How that can be?
In your first bullet you say the value becomes 00000010. That is incorrect. It actually becomes 000010 because of the RIGHT( , 6).

To clarify, compare the difference:

SELECT LEFT('00000010', 6), RIGHT('00000010', 6);


Results:

------  ------
000000  000010