Classement

Statistiques récapitulatives et fonctions de fenêtrage PostgreSQL

Michel Semaan

Data Scientist

Les fonctions de classement

ROW_NUMBER() attribue toujours des numéros uniques, même si les valeurs de deux lignes sont identiques
RANK() attribue le même numéro aux lignes présentant des valeurs identiques, en ignorant les numéros suivants dans de tels cas
DENSE_RANK() attribue également le même numéro aux lignes présentant des valeurs identiques, mais ne saute pas les numéros suivants

Tableau source

Requête

SELECT
  Country, COUNT(DISTINCT Year) AS Games
FROM Summer_Medals
WHERE
  Country IN ('GBR', 'DEN', 'FRA',
              'ITA', 'AUT', 'BEL',
              'NOR', 'POL', 'ESP')
GROUP BY Country
ORDER BY Games DESC;

Résultat

| Country | Games |
|---------|-------|
| GBR     | 27    |
| DEN     | 26    |
| FRA     | 26    |
| ITA     | 25    |
| AUT     | 24    |
| BEL     | 24    |
| NOR     | 22    |
| POL     | 20    |
| ESP     | 18    |

Différentes fonctions de classement - ROW_NUMBER

Requête

WITH Country_Games AS (...)

SELECT
  Country, Games,
  ROW_NUMBER()
    OVER (ORDER BY Games DESC) AS Row_N
FROM Country_Games
ORDER BY Games DESC, Country ASC;

Résultat

| Country | Games | Row_N |
|---------|-------|-------|
| GBR     | 27    | 1     |
| DEN     | 26    | 2     |
| FRA     | 26    | 3     |
| ITA     | 25    | 4     |
| AUT     | 24    | 5     |
| BEL     | 24    | 6     |
| NOR     | 22    | 7     |
| POL     | 20    | 8     |
| ESP     | 18    | 9     |

Différentes fonctions de classement - RANK

Requête

WITH Country_Games AS (...)

SELECT
  Country, Games,
  ROW_NUMBER()
    OVER (ORDER BY Games DESC) AS Row_N,
  RANK()
    OVER (ORDER BY Games DESC) AS Rank_N
FROM Country_Games
ORDER BY Games DESC, Country ASC;

Résultat

| Country | Games | Row_N | Rank_N |
|---------|-------|-------|--------|
| GBR     | 27    | 1     | 1      |
| DEN     | 26    | 2     | 2      |
| FRA     | 26    | 3     | 2      |
| ITA     | 25    | 4     | 4      |
| AUT     | 24    | 5     | 5      |
| BEL     | 24    | 6     | 5      |
| NOR     | 22    | 7     | 7      |
| POL     | 20    | 8     | 8      |
| ESP     | 18    | 9     | 9      |

Différentes fonctions de classement - DENSE_RANK

Requête

WITH Country_Games AS (...)

SELECT
  Country, Games,
  ROW_NUMBER()
    OVER (ORDER BY Games DESC) AS Row_N,
  RANK()
    OVER (ORDER BY Games DESC) AS Rank_N,
  DENSE_RANK()
    OVER (ORDER BY Games DESC) AS Dense_Rank_N
FROM Country_Games
ORDER BY Games DESC, Country ASC;

ROW_NUMBER et RANK auront le même rang final, le nombre de lignes

Résultat

| Country | Games | Row_N | Rank_N | Dense_Rank_N |
|---------|-------|-------|--------|--------------|
| GBR     | 27    | 1     | 1      | 1            |
| DEN     | 26    | 2     | 2      | 2            |
| FRA     | 26    | 3     | 2      | 2            |
| ITA     | 25    | 4     | 4      | 3            |
| AUT     | 24    | 5     | 5      | 4            |
| BEL     | 24    | 6     | 5      | 5            |
| NOR     | 22    | 7     | 7      | 5            |
| POL     | 20    | 8     | 8      | 6            |
| ESP     | 18    | 9     | 9      | 7            |

Le dernier rang de DENSE_RANK correspond au nombre de valeurs uniques classées

Classement sans partitionnement - Tableau source

Requête

SELECT
  Country, Athlete, COUNT(*) AS Medals
FROM Summer_Medals
WHERE
  Country IN ('CHN', 'RUS')
  AND Year = 2012
GROUP BY Country, Athlete
HAVING COUNT(*) > 1
ORDER BY Country ASC, Medals DESC;

Résultat

| Country | Athlete           | Medals |
|---------|-------------------|--------|
| CHN     | SUN Yang          | 4      |
| CHN     | Guo Shuang        | 3      |
| CHN     | WANG Hao          | 3      |
| ...     | ...               | ...    |
| RUS     | MUSTAFINA Aliya   | 4      |
| RUS     | ANTYUKH Natalya   | 2      |
| RUS     | ISHCHENKO Natalia | 2      |
| ...     | ...               | ...    |

Classement sans partitionnement

Requête

WITH Country_Medals AS (...)

SELECT
  Country, Athlete, Medals,
  DENSE_RANK()
    OVER (ORDER BY Medals DESC) AS Rank_N
FROM Country_Medals
ORDER BY Country ASC, Medals DESC;

Résultat

| Country | Athlete           | Medals | Rank_N |
|---------|-------------------|--------|--------|
| CHN     | SUN Yang          | 4      | 1      |
| CHN     | Guo Shuang        | 3      | 2      |
| CHN     | WANG Hao          | 3      | 2      |
| ...     | ...               | ...    | ...    |
| RUS     | MUSTAFINA Aliya   | 4      | 1      |
| RUS     | ANTYUKH Natalya   | 2      | 3      |
| RUS     | ISHCHENKO Natalia | 2      | 3      |
| ...     | ...               | ...    | ...    |

Classement avec partitionnement

Requête

WITH Country_Medals AS (...)

SELECT
  Country, Athlete,
  DENSE_RANK()
    OVER (PARTITION BY Country
              ORDER BY Medals DESC) AS Rank_N
FROM Country_Medals
ORDER BY Country ASC, Medals DESC;

Résultat

| Country | Athlete           | Medals | Rank_N |
|---------|-------------------|--------|--------|
| CHN     | SUN Yang          | 4      | 1      |
| CHN     | Guo Shuang        | 3      | 2      |
| CHN     | WANG Hao          | 3      | 2      |
| ...     | ...               | ...    | ...    |
| RUS     | MUSTAFINA Aliya   | 4      | 1      |
| RUS     | ANTYUKH Natalya   | 2      | 2      |
| RUS     | ISHCHENKO Natalia | 2      | 2      |
| ...     | ...               | ...    | ...    |

Passons à la pratique !

Statistiques récapitulatives et fonctions de fenêtrage PostgreSQL