مستوى صف SQL والربط والمصالحة

import sqlite3

db = sqlite3.connect(':memory:')
db.executescript('''
CREATE TABLE customer(id INTEGER PRIMARY KEY);
CREATE TABLE purchase(id INTEGER PRIMARY KEY, customer_id INTEGER);
INSERT INTO customer VALUES(1),(2);
INSERT INTO purchase VALUES(10,1),(11,1);
''')
rows = db.execute('''SELECT c.id,COUNT(p.id) FROM customer c
LEFT JOIN purchase p ON p.customer_id=c.id GROUP BY c.id ORDER BY c.id''').fetchall()
assert rows == [(1, 2), (2, 0)]
db.close()

النتيجة المحددة[(1,2),(2,0)] دون قاعدة قرص أو اتصال معد.

from itertools import product

lines = [10, 15]
payments = [5, 8, 12]
joined = list(product(lines, payments))
assert len(joined) == 6
assert sum(line for line, payment in joined) == 75
assert sum(payment for line, payment in joined) == 50
assert sum(lines) == sum(payments) == 25

تكرر الستة البند ثلاثاً والدفعة مرتين ويحفظ المستقل المعنى.