三段加起来才是整张表

把库存分成三段各数一次：等于 0 的、不等于 0 的、空着的，最后把三个数加起来。输出四个数（用 / 隔开）。

id  name    cat   price  stock
 1  绿茶    饮料      6     12
 2  红茶    饮料      6      0
 3  苏打水  饮料      5   (空)
 4  薯片    零食      8      7
 5  海苔    零食     12      3
 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, name TEXT, cat TEXT, price INTEGER, 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)])
def cnt(sql):
    return len(db.execute(sql).fetchall())

a = cnt("SELECT * FROM goods WHERE stock = 0")
b = cnt("SELECT * FROM goods WHERE stock <> 0")
c = cnt("SELECT * FROM goods WHERE stock IS NULL")
print(str(a) + "/" + str(b) + "/" + str(c) + "/" + str(a + b + c))