Find docs with "largest" value per (or grouped by) a category

SQL++
2 
SELECT specieskey ,  m[0] AS length, m[1] AS fishingtripkey
FROM data 
WHERE type='Catch' AND userkey='BA171123846CEBF1C1257CB2002DA330'
GROUP BY specieskey
LETTING m = MAX([length,fishingtripkey]);

Read Select non-group by fields using sql query for understanding the technique.