Select one row for each group

-- 1 Group by groupcol,
-- 2 Sort by sortcol
-- 3 Add numeration for every group

SELECT TABLE.*,
ROW_NUMBER() over (PARTITION BY TABLE.groupcol ORDER BY sortcol) AS rm -- set number for each group
FROM TABLE;

-- leave only one row for each group

SELECT * FROM
  (SELECT TABLE.*,
  ROW_NUMBER() over (PARTITION BY TABLE.groupcol ORDER BY sortcol) AS rm -- set number for each group
  FROM TABLE)
WHERE rm = 1;