如何從多對多mysql關系中選擇唯一值

Question

嗨，我在這個 sql 案例中感到困惑。 我正在使用mysql Ver 15.1 Distrib 10.2.31-MariaDB 。 假設我有 3 個表products 、 categories和product_categories作為數據透視表。

以下是數據示例：

products:

|    id    |   name    |
------------------------
| 1        | asd wef   |
| 2        | gggg2222  |
| 3        | pppga 99  |
| 4        | lalala 55 |

對於類別：

| id | level | parent_id | name     |
-----------------------------------
| 20 | 1     |           | Fashion  |
| 22 | 2     | 20        | Top      |
| 23 | 3     | 22        | T-Shirt  |
| 24 | 3     | 22        | Jacket   |

對於數據透視表，product_categories：

|  product_id  |  category_id  |
--------------------------------
| 1            | 20            |
| 1            | 22            |
| 1            | 23            |
| 2            | 22            |
| 2            | 20            |
| 3            | 20            |
| 4            | 20            |

因此，從數據透視表中可以看出，只有product_id = 3 & 4停止在類別級別 1 中。而product_id = 2僅停止在類別級別 2 中。

我想在這里實現的是當我從categories表中選擇時。 我可以數出有多少產品停在這里。 這是我想要獲取的數據示例。

"categories": [
    {
        "id": 20,
        "total_product": 4
        "stopped_product": 2
    },
    {
        "id": 22,
        "total_product": 3
        "stopped_product": 1
    }
]

到目前為止，我嘗試使用group by ：

SELECT * FROM product_categories WHERE product_id IN (1, 2, 3, 4) GROUP BY product_id HAVING category_id=20

輸出：

|  product_id  |  category_id  |
--------------------------------
| 1            | 20            |
| 3            | 20            |
| 4            | 20            |

預期產出

|  product_id  |  category_id  |
--------------------------------
| 3            | 20            |
| 4            | 20            |

Answer 1

我猜你只想要最大值

    SELECT product_id,MAX(category_id) as category_id FROM product_categories WHERE product_id IN (1, 2, 
    3, 4) 
    GROUP BY product_id 
    HAVING category_id=20

Answer 2

您可以使用 CTE 為每個產品生成停止類別列表，然后將其JOIN categories和product_categories表以計算每個類別的產品總數和停止產品數量：

WITH prods AS (
  SELECT product_id, MAX(category_id) AS stop_cat
  FROM product_categories
  GROUP BY product_id
)
SELECT c.id AS category_id, 
       COUNT(DISTINCT pc.product_id) AS total_product,
       SUM(c.id = p.stop_cat) AS stopped_product
FROM categories c
JOIN product_categories pc ON pc.category_id = c.id
JOIN prods p ON p.product_id = pc.product_id
GROUP BY c.id

輸出（對於您的樣本數據）

category_id     total_product   stopped_product
20              4               2
22              2               1
23              1               1

dbfiddle 上的演示

Answer 3

您可以在此處找到按類別分類的產品數量和 ID：

SELECT c.name as categoryName, c.level, COUNT(pc.product_id) as productCount, GROUP_CONCAT(pc.product_id) as productIds 
FROM product_categories pc JOIN categories c ON c.id = pc.category_id 
WHERE 1 
GROUP by c.id