SQL: How to perform string does not equal

Your where clause will return all rows where tester does not match username AND where tester is not null.

If you want to include NULLs, try:

where tester <> 'username' or tester is null

If you are looking for strings that do not contain the word "username" as a substring, then like can be used:

where tester not like '%username%'

Try the following query

select * from table
where NOT (tester = 'username')