Mysql 从表中选择值跨多行的位置

我有一个将产品映射到类别的表。每个产品可以属于多个类别，因此一个产品可以有多个具有不同类别ID的行。它看起来像这样：

CREATE TABLE `products_mid` (
  `id` int(11) UNSIGNED NOT NULL,
  `product_id` int(11) UNSIGNED NOT NULL DEFAULT '0',
  `category_id` int(11) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

SELECT product_id
GROUP BY product_id
HAVING SUM(CASE WHEN category_id=1 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=2 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=3 THEN 1 ELSE 0 END) > 0

select product_id from products_mid
where
category_id in (1,2,3)
group by product_id
having count(distinct category_id)=3

---

如果我想得到一个属于类别a的所有产品ID的列表，这很简单，但是有没有一种方法可以尽可能简单地得到属于类别a、B和C的所有产品ID？

更好的方法是使用have子句，而不是where子句。按product_id分组，并选择三个类别中计数均为非零的行，如下所示：

CREATE TABLE `products_mid` (
  `id` int(11) UNSIGNED NOT NULL,
  `product_id` int(11) UNSIGNED NOT NULL DEFAULT '0',
  `category_id` int(11) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

SELECT product_id
GROUP BY product_id
HAVING SUM(CASE WHEN category_id=1 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=2 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=3 THEN 1 ELSE 0 END) > 0

select product_id from products_mid
where
category_id in (1,2,3)
group by product_id
having count(distinct category_id)=3

---

上面的语句选择了属于ID为1、2和3的所有三个类别的产品的产品ID。

更好的方法是使用HAVING子句，而不是WHERE子句。按product_id分组，并选择三个类别中计数均为非零的行，如下所示：

CREATE TABLE `products_mid` (
  `id` int(11) UNSIGNED NOT NULL,
  `product_id` int(11) UNSIGNED NOT NULL DEFAULT '0',
  `category_id` int(11) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

SELECT product_id
GROUP BY product_id
HAVING SUM(CASE WHEN category_id=1 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=2 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=3 THEN 1 ELSE 0 END) > 0

select product_id from products_mid
where
category_id in (1,2,3)
group by product_id
having count(distinct category_id)=3

上面的语句选择属于ID为1、2和3的所有三个类别的产品的产品ID。

另一种方式：

SELECT product_id
FROM products_mid
WHERE category_id IN (1, 2, 3)
GROUP BY product_id
HAVING COUNT(DISTINCT category_id) = 3

这将找到类别1到3中的所有产品，尽管产品也可能属于其他类别。如果您想要的产品只属于类别1到类别3，而不属于其他类别，那么我们必须重新表述查询。

另一种方式：

SELECT product_id
FROM products_mid
WHERE category_id IN (1, 2, 3)
GROUP BY product_id
HAVING COUNT(DISTINCT category_id) = 3

---

这将找到类别1到3中的所有产品，尽管产品也可能属于其他类别。如果您想要的产品只属于类别1到类别3，而不属于其他类别，那么我们必须重新表述查询。

它将是这样的：

CREATE TABLE `products_mid` (
  `id` int(11) UNSIGNED NOT NULL,
  `product_id` int(11) UNSIGNED NOT NULL DEFAULT '0',
  `category_id` int(11) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

SELECT product_id
GROUP BY product_id
HAVING SUM(CASE WHEN category_id=1 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=2 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=3 THEN 1 ELSE 0 END) > 0

select product_id from products_mid
where
category_id in (1,2,3)
group by product_id
having count(distinct category_id)=3

---

它将是这样的：

CREATE TABLE `products_mid` (
  `id` int(11) UNSIGNED NOT NULL,
  `product_id` int(11) UNSIGNED NOT NULL DEFAULT '0',
  `category_id` int(11) UNSIGNED NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

SELECT product_id
GROUP BY product_id
HAVING SUM(CASE WHEN category_id=1 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=2 THEN 1 ELSE 0 END) > 0
   AND SUM(CASE WHEN category_id=3 THEN 1 ELSE 0 END) > 0

select product_id from products_mid
where
category_id in (1,2,3)
group by product_id
having count(distinct category_id)=3

使用intersect运算符查找三个单独查询的公共值

select product_id from products_mid where category_id = 1
intersect
select product_id from products_mid where category_id = 2
intersect
select product_id from products_mid where category_id = 3

使用intersect运算符查找三个单独查询的公共值

select product_id from products_mid where category_id = 1
intersect
select product_id from products_mid where category_id = 2
intersect
select product_id from products_mid where category_id = 3

---

注意：intersect在MySQL中不可用，因此对于这个特定问题不是一个好答案，但我保留它是因为几乎所有其他数据库系统都支持它。注意：intersect在MySQL中不可用，因此对于这个特定问题不是一个好答案，但我保留它是因为几乎所有其他数据库系统都支持它。此表中的“代理项id”列似乎是多余的，因为您在其余列上似乎具有令人满意的自然键。此表中的“代理项id”列似乎是多余的，因为您似乎在其余列上具有完全令人满意的自然键。