一张完整的分类报表

按类别出一张报表：每一类的样数、总价、均价（保留一位小数），按类别名排好。输出三行（组内用 : 连，组间用 / 隔开）。

id  name    cat   price  stock      约束
 1  绿茶    饮料      6     12      id    主键
 2  红茶    饮料      6      0      name  非空且唯一
 3  苏打水  饮料      5   (空)      cat   非空
 4  薯片    零食      8      7      price 非空
 5  海苔    零食     12      3      stock 可以为空
 6  巧克力  零食     15   (空)
 7  铅笔    文具      2     40
 8  笔记本  文具      9      5
 9  橡皮    文具      3      0
10  冰红茶  饮料      4     20

import sqlite3
db = sqlite3.connect(":memory:")
db.execute("""CREATE TABLE goods (
    id    INTEGER PRIMARY KEY,
    name  TEXT NOT NULL UNIQUE,
    cat   TEXT NOT NULL,
    price INTEGER NOT NULL,
    stock INTEGER)""")
db.executemany("INSERT INTO goods VALUES (?,?,?,?,?)", [
    (1, "绿茶", "饮料", 6, 12),   (2, "红茶", "饮料", 6, 0),
    (3, "苏打水", "饮料", 5, None), (4, "薯片", "零食", 8, 7),
    (5, "海苔", "零食", 12, 3),   (6, "巧克力", "零食", 15, None),
    (7, "铅笔", "文具", 2, 40),   (8, "笔记本", "文具", 9, 5),
    (9, "橡皮", "文具", 3, 0),    (10, "冰红茶", "饮料", 4, 20)])
rows = db.execute("SELECT cat, COUNT(*), SUM(price), ROUND(AVG(price),1) "
                  "FROM goods GROUP BY cat ORDER BY cat").fetchall()
print("/".join(r[0] + ":" + str(r[1]) + ":" + str(r[2]) + ":" + str(r[3])
                for r in rows))