SQL 子集查询

Question

我在为 SQL table 创建查询时遇到问题。我尝试创建的查询显示 "clothes" 类别中的产品数量，但不显示配饰，例如作为 T 恤或运动衫输入的产品列表。

这是已创建的 table：

CREATE DATABASE IF NOT EXISTS product_list;

DROP TABLE IF EXISTS products;
DROP TABLE IF EXISTS product_categories;
DROP TABLE IF EXISTS categories;

CREATE TABLE products (
    product_id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(50) DEFAULT NULL,
    active BOOL DEFAULT NULL
);

CREATE TABLE categories (
    category_id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(50),
    structure VARCHAR(50)
);

CREATE TABLE product_categories (
    product_id INT,
    category_id INT,
    PRIMARY KEY(product_id, category_id)
);

INSERT INTO products VALUES
(NULL, "Blue Sweatshirt", false),
(NULL, "Short Sleeve T-Shirt", true),
(NULL, "White Vest", true),
(NULL, "Black Hairclip", true),
(NULL, "Knitted Hat", false),
(NULL, "Grey Sweatshirt", true),
(NULL, "Tartan Scarf", true);

INSERT INTO categories VALUES
(NULL, "Sweatshirts", "Clothes>Sweatshirts"),
(NULL, "T-Shirts", "Clothes>T-Shirts"),
(NULL, "Accessories", "Accessories"),
(NULL, "Winter", "Clothes>Winter"),
(NULL, "Vests", "Clothes>Vests");

INSERT INTO product_categories VALUES
(1, 1), (2, 2), (3, 5), (3, 4), (4, 3), (5, 3), (5, 4), (6, 1), (7, 3), (7, 4);

Answer 1

试试这个查询

select * from products a
join Product_categories b on a.product_id=b.product_id
join categories c on b.category_id=b.category_id
where c.name like '%Clothes%'

Answer 2

如果我没理解错的话，这是一个集合中的查询。您正在寻找至少具有一个 "clothes" 类别的产品，其中 none 个类别不是衣服。我使用 group by 和 having 来解决这个问题，因为它非常灵活：

select pc.product_id
from Product_categories pc join
     categories c
     on pc.category_id = c.category_id
group by pc.product_id
having sum(case when c.structure like 'Clothes%' then 1 else 0 end) > 0 and
       sum(case when c.structure not like 'Clothes%' then 1 else 0 end) = 0;

如果你只想要计数，那么你可以将其用作子查询并使用 count(*).

编辑：

一个小纸条。这个问题现在用 MySQL 标记，它是 having 子句的方便简写：

having sum(c.structure like 'Clothes%') > 0 and
       sum(c.structure not like 'Clothes%') = 0;

SQL 子集查询

SQL subsets query

mysql

sql

subquery