如何用python[duplicate]更新sqlite3中的记录

这里,在更新部分,将创建一个新的副本,而不是更新现有记录。请帮忙!

def addBooks(title, quantity):
    c.execute('SELECT * FROM books')
    for i, j in c.fetchall():
        if i == title:
            c.execute('UPDATE books SET quantity = ? WHERE title = ?', (j+quantity, title))
            conn.commit()
        else:
            c.execute('INSERT INTO books VALUES (?, ?)', (title, quantity))
            conn.commit()