Sous-requêtes dans SELECT

Manipulation de données en SQL

Mona Khalil

Data Scientist, Greenhouse Software

Fonction SELECT

Renvoie une valeur unique
- Inclure les valeurs agrégées afin de les comparer aux valeurs individuelles

Employée dans les calculs mathématiques
- Écart par rapport à la moyenne

Sous-requêtes dans SELECT

Calculer le nombre total de matchs sur toutes les saisons

SELECT COUNT(id) FROM match;

Sous-requêtes dans SELECT

SELECT
  season,
  COUNT(id) AS matches,
  12837 as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

Sous-requêtes dans SELECT

SELECT
  season,
  COUNT(id) AS matches,
  (SELECT COUNT(id) FROM match) as total_matches
FROM match
GROUP BY season;

| season    | matches | total_matches |
|-----------|---------|---------------|
| 2011/2012 | 3220    | 12837         |
| 2012/2013 | 3260    | 12837         |
| 2013/2014 | 3032    | 12837         |
| 2014/2015 | 3325    | 12837         |

SELECT sous-requêtes pour les calculs mathématiques

SELECT AVG(home_goal + away_goal) 
FROM match
WHERE season = '2011/2012';

2.72

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 2.72 AS diff
FROM match
WHERE season = '2011/2012';

Sous-requêtes dans SELECT

SELECT
  date,
  (home_goal + away_goal) AS goals,
  (home_goal + away_goal) - 
     (SELECT AVG(home_goal + away_goal) 
      FROM match
      WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

| date       | goals | diff              |
|------------|-------|-------------------|
| 2011-07-29 | 3     | 0.28354037267081  |
| 2011-07-30 | 2     | -0.71645962732919 |
| 2011-07-30 | 4     | 1.28354037267081  |
| 2011-07-30 | 1     | -1.71645962732919 |

SELECT sous-requêtes - éléments importants

Nécessaire de renvoyer une valeur UNIQUE
- Générera une erreur dans le cas contraire
S’assurer que tous les filtres sont correctement appliqués
- Bien filtrer requête principale et sous-requête !

SELECT
    date,
    (home_goal + away_goal) AS goals,
    (home_goal + away_goal) - 
       (SELECT AVG(home_goal + away_goal) 
        FROM match
        WHERE season = '2011/2012') AS diff
FROM match
WHERE season = '2011/2012';

Passons à la pratique !

Manipulation de données en SQL