I hope the following helps you:
var_found = 22; % variable found in database
%example struct
s.name='ww';
s.age=22;
n=fieldnames(s); % fieldnames within the struct
for k=1:numel(n)
if s.(n{k})==var_found
wanted_field = n{k}; % the field containing the value found in the database
break
end
end
