Tabellen combineren

Rapporteren in SQL

Tyler Pernes

Learning & Development Consultant

Doelrapport

Gouden medailles per demografische groep
(Alleen West-Europese landen)
+----------+--------------------+-------+
| season   |  demographic_group | golds |
|----------+--------------------+-------|
| Winter   | Male Age 26+       | 13    |
| Winter   | Female Age 26+     | 8     |
| Summer   | Male Age 13-25     | 7     |
| Summer   | Female Age 13-25   | 6     |
| Winter   | Male Age 13-25     | 4     |
| Summer   | Male Age 26+       | 4     |
| Winter   | Female Age 13-25   | 4     |
| Summer   | Female Age 26+     | 2     |
+----------+--------------------+-------+

Relevante tabellen

Optie A: eerst JOIN, daarna UNION

Stap 1: Bovenste query opzetten met JOIN

SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM summer_games AS sg
JOIN athletes AS a 
ON sg.athlete_id = a.id;

Query succesvol uitgevoerd!

Optie A: eerst JOIN, daarna UNION

Stap 2: Onderste query opzetten + beide {{1}} UNION-en

SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM summer_games AS sg
JOIN athletes AS a 
ON sg.athlete_id = a.id
UNION ALL
SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM winter_games AS wg
JOIN athletes AS a 
ON wg.athlete_id = a.id;

Optie B: eerst UNION, daarna JOIN

Stap 1: Eerste UNION maken

SELECT 
    athlete_id, 
    gold  
FROM summer_games AS sg
UNION
SELECT 
    athlete_id, 
    gold  
FROM winter_games AS wg;

Optie B: eerst UNION, daarna JOIN

Stap 2: Omzetten naar subquery + JOIN

SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM
    (SELECT 
         athlete_id, 
         gold  
    FROM summer_games AS sg
    UNION ALL
    SELECT athlete_id, gold  
    FROM winter_games AS wg) AS g
JOIN athletes AS a 
ON g.athlete_id = a.id;

Vergelijking

Optie A

SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM summer_games AS sg
JOIN athletes AS a 
ON sg.athlete_id = a.id
UNION ALL
SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM winter_games AS wg
JOIN athletes AS a 
ON wg.athlete_id = a.id;

Optie B

SELECT 
    athlete_id, 
    gender, 
    age, 
    gold  
FROM
    (SELECT 
         athlete_id, 
         gold  
    FROM summer_games AS sg
    UNION ALL
    SELECT athlete_id, gold  
    FROM winter_games AS wg) AS g
JOIN athletes AS a 
ON g.athlete_id = a.id;

Belangrijkste punten

Meerdere manieren voor hetzelfde rapport
Stap-voor-stap = makkelijker te debuggen

Tijd om te query'en!

Rapporteren in SQL