create index registrants_lname_asc on registrants (lname ASC);

SET @rownum := 0
SELECT CONCAT(min(@rownum := @rownum + 1), '-',
              max(@rownum), ' (', 
              min(lname), '-', max(lname), ')')
  from registrants use index (registrants_lname_asc)
 group by floor(@rownum/100)