SELECT MIN(r.i2) FROM ... WHERE ... AND r.i2>0
UNION
SELECT MIN(g.i2) FROM ... WHERE ... AND g.i2>0
ORDER BY 1 ASC LIMIT 1

SELECT MIN(r.i2) FROM ... WHERE ... AND r.i2>0
UNION
SELECT MIN(g.i2) FROM ... WHERE ... AND g.i2>0

NULL
599

...
ORDER BY 1 ASC LIMIT 1

SELECT LEAST(IFNULL(NULLIF(a,0),b),(IFNULL(NULLIF(b,0),a))) ID
FROM (
   SELECT MIN(IF(r.i2>0,r.i2,NULL))a,MIN(IF(g.i2>0,g.i2,NULL))b
   FROM records_db s
   JOIN records_db g on g.base = 124 and g.g2 = s.id
   JOIN records_db c on c.base = 125 and c.g1 = g.id
   JOIN records_db r on r.base = 126 and r.g1 = c.id
   WHERE s.base = 123 and s.id = АЙДИ
   )q