Frame

Statistiche riepilogative e funzioni finestra in PostgreSQL

Michel Semaan

Data Scientist

Motivazione

LAST_VALUE

LAST_VALUE(City) OVER (
 ORDER BY Year ASC
 RANGE BETWEEN
   UNBOUNDED PRECEDING AND
   UNBOUNDED FOLLOWING
) AS Last_City

Frame: RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
Senza frame, LAST_VALUE restituisce il valore della riga nella colonna City
Per impostazione predefinita, un frame inizia all'inizio della tabella o partizione e finisce alla riga corrente

ROWS BETWEEN

ROWS BETWEEN [START] AND [FINISH]
- n PRECEDING: n righe prima della riga corrente
- CURRENT ROW: la riga corrente
- n FOLLOWING: n righe dopo la riga corrente

Esempi

ROWS BETWEEN 3 PRECEDING AND CURRENT ROW
ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
ROWS BETWEEN 5 PRECEDING AND 1 PRECEDING

Tabella di origine

Query

SELECT
  Year, COUNT(*) AS Medals
FROM Summer_Medals
WHERE
  Country = 'RUS'
  AND Medal = 'Gold'
GROUP BY Year
ORDER BY Year ASC;

Risultato

| Year | Medals |
|------|--------|
| 1996 | 36     |
| 2000 | 66     |
| 2004 | 47     |
| 2008 | 43     |
| 2012 | 47     |

MAX senza frame

Query

WITH Russia_Medals AS (...)

SELECT
  Year, Medals,
  MAX(Medals)
    OVER (ORDER BY Year ASC) AS Max_Medals
FROM Russia_Medals
ORDER BY Year ASC;

Risultato

| Year | Medals | Max_Medals |
|------|--------|------------|
| 1996 | 36     | 36         |
| 2000 | 66     | 66         |
| 2004 | 47     | 66         |
| 2008 | 43     | 66         |
| 2012 | 47     | 66         |

MAX con frame

Query

WITH Russia_Medals AS (...)

SELECT
  Year, Medals,
  MAX(Medals)
    OVER (ORDER BY Year ASC) AS Max_Medals,
  MAX(Medals)
    OVER (ORDER BY Year ASC
          ROWS BETWEEN
          1 PRECEDING AND CURRENT ROW)
    AS Max_Medals_Last
FROM Russia_Medals
ORDER BY Year ASC;

Risultato

| Year | Medals | Max_Medals | Max_Medals_Last |
|------|--------|------------|-----------------|
| 1996 | 36     | 36         | 36              |
| 2000 | 66     | 66         | 66              |
| 2004 | 47     | 66         | 66              |
| 2008 | 43     | 66         | 47              |
| 2012 | 47     | 66         | 47              |

Riga corrente e successiva

Query

WITH Russia_Medals AS (...)

SELECT
  Year, Medals,
  MAX(Medals)
    OVER (ORDER BY Year ASC
          ROWS BETWEEN
          CURRENT ROW AND 1 FOLLOWING)
    AS Max_Medals_Next
FROM Russia_Medals
ORDER BY Year ASC;

Risultato

| Year | Medals | Max_Medals_Next |
|------|--------|-----------------|
| 1996 | 36     | 66              |
| 2000 | 66     | 66              |
| 2004 | 47     | 47              |
| 2008 | 43     | 47              |
| 2012 | 47     | 47              |

Passons à la pratique !

Statistiche riepilogative e funzioni finestra in PostgreSQL