MAX() in WHERE clause
SELECT stel.SYM, stel.RT1MONTH, val10kst.VAL10K, val10kst.VAL10KID, val10kst.UPDATED
FROM stel
LEFT JOIN val10kst on val10kst.TICKER=stel.SYM
WHERE val10kst.VALDATE=(SELECT MAX(val10kst.VALDATE) FROM val10kst) AND stel.SYM !=''