SQL optymalizacja

a.txt

Znajdź dla każdego ze studentów 1,2,3,4 osobno
taki ich rekord LPL, który został stworzony jako
ostatni przed dniem 2013-02-28

b.sql

SELECT    lpl.*
FROM      lesson_progress_levels AS lpl
LEFT JOIN lesson_progress_levels AS lpl_after
ON        lpl_after.created_at > lpl.created_at AND
          lpl_after.created_at < '2013-03-01 00:00' AND
          lpl_after.student_id = lpl.student_id
WHERE     lpl_after.id IS NULL AND
          lpl.created_at < '2013-03-01 00:00' AND
          lpl.student_id IN (1,2,3,4)

d.sql

SELECT oute.*
FROM lesson_progress_levels AS oute
JOIN (
  SELECT    student_id, lesson_id, max(created_at) as mm
  FROM      lesson_progress_levels AS lpl
  WHERE     lpl.created_at < '2013-03-01 00:00' AND
            lpl.student_id IN (1,2,3,4)
  GROUP BY  lpl.student_id, lpl.lesson_id
) as ine ON 
  oute.student_id = ine.student_id AND
  oute.lesson_id  = ine.lesson_id  AND
  oute.created_at = ine.mm

e.sql

  SELECT    student_id, lesson_id, max(id) as id
  FROM      lesson_progress_levels AS lpl
  WHERE     lpl.created_at < '2013-03-01 00:00' AND
            lpl.student_id IN (1,2,3,4)
  GROUP BY  lpl.student_id, lpl.lesson_id

schema.rb

  create_table "lesson_progress_levels", :force => true do |t|
    t.integer  "lesson_id"
    t.integer  "student_id"
    t.integer  "training_id"
    t.decimal  "level",       :precision => 5, :scale => 2, :default => 0.0
    t.datetime "created_at"
    t.datetime "updated_at"
  end

tmp_c.sql

SELECT    student_id, lesson_id, max(created_at) as mm
FROM      lesson_progress_levels AS lpl
WHERE     lpl.created_at < '2013-03-01 00:00' AND
          lpl.student_id IN (1,2,3,4)
GROUP BY  lpl.student_id, lpl.lesson_id
HAVING    mm = max(created_at)

wrong_1.sql

SELECT    lpl.*
FROM      lessons_progress_level AS lpl
LEFT JOIN lessons_progress_level AS lpl_before
ON        lpl_before.created_at < lpl.created_at AND
          lpl_before.student_id = lpl.student_id
WHERE     lpl_before.id IS NULL AND
          lpl.created_at < '2013-02-28' AND
          lpl.student_id IN (1,2,3,4)