SELECT gb, seq
     , @flag := CASE WHEN gb  = @gb AND seq  = @seq     THEN '중복'
                     WHEN gb  = @gb AND seq != @seq + 1 THEN '누락'
                     WHEN gb != @gb AND seq != 1        THEN '누락'
                 END AS flag
     , @gb  := gb
     , @seq := seq
  FROM (SELECT 1 gb, 1 seq
        UNION ALL SELECT 1, 2
        UNION ALL SELECT 2, 1
        UNION ALL SELECT 2, 2
        UNION ALL SELECT 1, 4
        UNION ALL SELECT 1, 4
         ORDER BY gb, seq
        ) a
     , (SELECT @gb := 0, @seq := 0, @flag := '') b
;