with sqlite3.connect(':memory:') as db:
    db.executescript('CREATE TABLE orders(sku TEXT,qty INT); CREATE TABLE stocks(sku TEXT,stock INT);')
    db.executemany('INSERT INTO orders VALUES(?,?)', [(row['sku'], row['qty']) for row in data['orders']])
    db.executemany('INSERT INTO stocks VALUES(?,?)', [(row['sku'], row['stock']) for row in data['stocks']])
    joined = db.execute('SELECT orders.sku,orders.qty,stocks.stock FROM orders LEFT JOIN stocks USING(sku) ORDER BY orders.sku').fetchall()
check('模型关联结果与真实SQL LEFT JOIN一致', [(row['sku'], row['ordered'], row['stock']) for row in output['rows']] == joined)
save_csv(folder / 'joined.csv', output['rows'])
