SQL query to add Zero prefix

Let say,
I have a Database column 'ISSN' and data in it, should be 8 digits long. When I import data from excel to SQL table, it truncates the initial 1-3 zeros because it doesn't like zero as first digits when you mark the column as int but if you mark as text that's fine.
Now, let's write SQL query so that if the ISSN is 5 digits long then add 3 zeros to prefix.
e.g If the ISSN is 45678 then it should be '00045678'
If it is 6 digits long, then add two zeros to prefix....If 8 digits long keep as it is.

Hence, the SQL query is:

update table
set ISSN = RIGHT('00000000'+ISSN,8)

Happy Programming !!!