MySQL动态行转列操作

问题：

从多个表中检索数据，并将其格式化为动态列标题的透视表，即使事先不知道确切的列数。

原始查询：

SELECT partners.name, products.name, COUNT(*) 
FROM sales
JOIN products ON sales.products_id = products.id
JOIN partners ON sales.partners_id = partners.id
GROUP BY sales.partners_id, sales.products_id
LIMIT 0, 30

使用CASE语句的解决方案：

MySQL没有PIVOT函数，因此需要使用带有CASE语句的聚合函数：

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

使用预处理语句的动态透视表：

为了处理动态列标题，可以使用预处理语句：

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;