Samozłączenia

Łączenie danych w SQL

Maham Faisal Khan

Senior Content Developer, DataCamp

Samozłączenia

Samozłączenia to złączenia tabeli z samą sobą
Służą do porównywania części tej samej tabeli

Tabela prime_ministers z bazy danych o przywódcach świata

Tabela prime_ministers z bazy danych o przywódcach świata, zawierająca trzy kolumny: country, continent i prime_minister.

Premier poznaje premiera

SELECT 
    p1.country AS country1, 
    p2.country AS country2, 
    p1.continent
FROM prime_ministers AS p1
INNER JOIN prime_ministers AS p2

ON p1.continent = p2.continent
LIMIT 10;

Premier poznaje premiera

| country1   | country2   | continent     |
|------------|------------|---------------|
| Egypt      | Egypt      | Africa        |
| Portugal   | Spain      | Europe        |
| Portugal   | Norway     | Europe        |
| Portugal   | Portugal   | Europe        |
| Vietnam    | Oman       | Asia          |
| Vietnam    | Brunei     | Asia          |
| Vietnam    | India      | Asia          |
| Vietnam    | Vietnam    | Asia          |
| Haiti      | Haiti      | North America |
| India      | Oman       | Asia          |
| India      | Brunei     | Asia          |

Premier poznaje premiera

SELECT 
    p1.country AS country1, 
    p2.country AS country2, 
    p1.continent
FROM prime_ministers AS p1
INNER JOIN prime_ministers AS p2
ON p1.continent = p2.continent 

    AND p1.country <> p2.country;

Tabela premierów po samozłączeniu

|------------|------------|-------------|
| country1   | country2   | continent   |
|------------|------------|-------------|
| Portugal   | Spain      | Europe      |
| Portugal   | Norway     | Europe      |
| Vietnam    | Oman       | Asia        |
| Vietnam    | Brunei     | Asia        |
| Vietnam    | India      | Asia        |
| India      | Oman       | Asia        |
| India      | Brunei     | Asia        |
| India      | Vietnam    | Asia        |
| Norway     | Spain      | Europe      |
| Norway     | Portugal   | Europe      |
|------------|------------|-------------|

Czas na ćwiczenia!

Łączenie danych w SQL