SELECT id, MIN(i2)
FROM `records` as `tbl1`
WHERE base = 126
  and g1 in (
    SELECT id
    FROM `records`
    WHERE base = 125
      and g1 in (
        SELECT id
        FROM `records`
        WHERE base = 124
          and g2 in (
            SELECT id
            FROM `records`
            WHERE base = 123
              and g1 = 34845
)))

select r.id, r.i126, r.i124, if(r.i124<r.i126,r.i124,r.i126)i2
from (
   select r.id, min(r.i2) i126, min(g.i124)i124
   from (
      SELECT r.id, MIN(r.i2) i124
      FROM records c
      JOIN records r on r.base=124 and r.g2=c.id
      where c.base = 123 and c.g1=34845
      group by r.id
   ) g
   JOIN records c on c.base=125 and c.g1=g.id
   join records r on r.base=126 and r.g1=c.id
   group by r.id
   ) r;