import  sqlite3
conn = sqlite3.connect('food.db')
c = conn.cursor()

c.execute('''CREATE TABLE IF NOT EXISTS nutritional_values
                (item, calories, total fat, protein)''')
items = [       ('Broccoli Chinese',    22,     0.7,    1.1),
                ('chia seeds',          490,    30.8,   15.6),
                ('blueberries',         57,     0.3,    0.7),]
c.executemany('INSERT INTO nutritional_values VALUES (?,?,?,?)',items)


c.execute('''CREATE TABLE IF NOT EXISTS food_consumption
                (date, item, amount)''')
c.execute('DELETE FROM food_consumption')
consumed = [    ('24/8/2019', 'Broccoli Chinese',    1.5),
                ('24/8/2019', 'chia seeds',          0.35),
                ('24/8/2019', 'blueberries',         0.4),]
c.executemany('INSERT INTO food_consumption VALUES (?,?,?)',consumed)

conn.commit()
conn.close()

如果我跑

for row in c.execute('SELECT * FROM food_consumption'):
    print(row[1])

正如所料，我得到了

Broccoli Chinese
chia seeds
blueberries

但如果我跑

for row in c.execute('SELECT * FROM food_consumption'):
    print(row[1])
    for number in c.execute('SELECT calories FROM nutritional_values WHERE item=(?)', (row[1],)):
        print(number)

我明白了

Broccoli Chinese
(22,)

所以它不再遍历表'food_consumption'中的所有行，我如何让它循环遍历所有行。

分析解答

我不知道为什么你当前的方法不起作用，但无论如何它是一个糟糕的设计模式，你应该通过在两个表之间进行连接，处理数据库端的关系：

sql = """SELECT nv.item, nv.calories
         FROM nutritional_values nv
         INNER JOIN food_consumption fc
             ON nv.item = fc.item"""

for row in c.execute(sql):
    print(row[1])

请注意，这是处理问题的首选方法，因为它可以避免将整个结果集加载到Python的内存中。