There is a notes field in core_occurrence_attendance that we would sometimes use where attended may still = 0.
For example, we recently held a class which we also made available online and we marked those that attended in person as attended, but in the notes field entered "online" if they told us they watched the class online. Our attendance chart report marks any notes as an asterisk in the grid and then lists the notes below.
So, if you wanted to clear out those entries but keep anything with notes, I would say:
WHERE attended=0 AND notes=''
