MySQL – Alternativen zu verschachtelten Unterabfragen beim Einschränken von aggregierten Daten in einer korrelierten Unterabfrage

Nein, Sie können keine korrelierten Unterabfragen in FROM verwenden Klausel. Aber Sie können sie im ON verwenden Bedingungen:

SELECT AVG(d.DailyData1) Data1_20DayAvg 
       --- other aggregate stuff on d (Datatable)
FROM 
      ( SELECT '2012-01-23' AS DateChecked
      ) AS dd
  JOIN
      DataTable AS d
    ON
      d.Date <= dd.DateChecked
    AND
      d.Date >= COALESCE( 
      ( SELECT DailyData1 
        FROM DataTable AS last20 
        WHERE Date <= dd.DateChecked 
          AND (other conditions for last20)
        ORDER BY Date DESC 
        LIMIT 1 OFFSET 19
      ), '1001-01-01'   )
WHERE (other conditions for d Datatable)

Ähnlich für viele Daten:

SELECT dd.DateChecked 
     , AVG(d.DailyData1) Data1_20DayAvg 
       --- other aggregate stuff on d (Datatable)
FROM 
      ( SELECT DISTINCT Date AS DateChecked
        FROM DataTable 
      ) AS dd
  JOIN
      DataTable AS d
    ON
      d.Date <= dd.DateChecked
    AND
      d.Date >= COALESCE( 
      ( SELECT DailyData1 
        FROM DataTable AS last20
        WHERE Date <= dd.DateChecked 
          AND (other conditions for last20)
        ORDER BY Date DESC 
        LIMIT 1 OFFSET 19
      ), '1001-01-01'   )
WHERE (other conditions for d Datatable)
GROUP BY 
      dd.DateChecked 

Beide Abfragen gehen davon aus, dass Datatable.Date hat einen UNIQUE Einschränkung.