Pythonda bir jadval bilan ikki tomonlama bog‘lanish
Faraz qilaylik, bizda shaharlar mavjud:
| id | name |
|---|---|
| 1 | city1 |
| 2 | city2 |
| 3 | city3 |
Faraz qilaylik, bizda shaharlar orasida marshrutlar mavjud, bunda har bir marshrut boshlanish shahri va tugash shahriga ega:
| id | name | from_city_id | to_city_id |
|---|---|---|---|
| 1 | route1 | 1 | 2 |
| 2 | route2 | 2 | 3 |
Faraz qilaylik, biz shaharlar bilan birga marshrutlarni olishni xohlaymiz. Bu yerda murakkablik shundaki, har bir marshrut ikkita shaharga ega: boshlanish va tugash.
Ma'lum bo'ladiki, shaharlar jadvalini ikki marta join qilish kerak. Har bir join paytida jadvalni qayta nomlash kerak bo'ladi:
SELECT
from_cities.name as from_city_name,
to_cities.name as to_city_name
FROM
routes
LEFT JOIN cities as from_cities
ON from_cities.id=routes.from_city_id
LEFT JOIN cities as to_cities
ON to_cities.id=routes.to_city_id
Faraz qilaylik, bizda foydalanuvchilar mavjud. Har bir foydalanuvchi otasi va onasiga ega. Saqlash tuzilmasini tavsiflang.
