如何將 MySQL 行轉換為動態數量的欄位？

MySQL 行轉列：動態列數的實作

在 MySQL 中，即使列數未知，你也可以有效地將行轉換為列，方法是結合聚合函數和動態查詢。

使用 CASE 語句的 Pivot 查詢

要將問題中提到的表格進行透視，請使用帶有 CASE 語句的 COUNT() 函數：

SELECT pt.partner_name,
  COUNT(CASE WHEN pd.product_name = 'Product A' THEN 1 END) AS ProductA,
  COUNT(CASE WHEN pd.product_name = 'Product B' THEN 1 END) AS ProductB,
  COUNT(CASE WHEN pd.product_name = 'Product C' THEN 1 END) AS ProductC,
  COUNT(CASE WHEN pd.product_name = 'Product D' THEN 1 END) AS ProductD,
  COUNT(CASE WHEN pd.product_name = 'Product E' THEN 1 END) AS ProductE
FROM partners pt
LEFT JOIN sales s ON pt.part_id = s.partner_id
LEFT JOIN products pd ON s.product_id = pd.prod_id
GROUP BY pt.partner_name;

使用預處理語句的動態 Pivot

為了根據產品數量動態產生 Pivot 查詢，可以使用預處理語句：

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'COUNT(CASE WHEN Product_Name = ''',
      Product_Name,
      ''' THEN 1 END) AS ',
      REPLACE(Product_Name, ' ', '')
    )
  ) INTO @sql
FROM products;

SET @sql = CONCAT('SELECT pt.partner_name, ', @sql, ' FROM partners pt
LEFT JOIN sales s ON pt.part_id = s.partner_id
LEFT JOIN products pd ON s.product_id = pd.prod_id
GROUP BY pt.partner_name');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

此查詢根據 products 表動態建立 SQL 語句，確保結果中的列數與產品數相符。

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