MySQL pivot row成动态列数

Question

可以说我有三个不同的MySQL表:

表products:

id | name
 1   Product A
 2   Product B

表partners:

id | name
 1   Partner A
 2   Partner B

表sales:

partners_id | products_id
          1             2
          2             5
          1             5
          1             3
          1             4
          1             5
          2             2
          2             4
          2             3
          1             1

我想得到一个表格,其中包含行和产品中的合作伙伴作为列.到目前为止,我能够得到这样的输出:

name      | name      | COUNT( * )
Partner A   Product A          1
Partner A   Product B          1
Partner A   Product C          1
Partner A   Product D          1
Partner A   Product E          2
Partner B   Product B          1
Partner B   Product C          1
Partner B   Product D          1
Partner B   Product E          1

使用此查询:

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

但我希望有类似的东西:

partner_name | Product A | Product B | Product C | Product D | Product E
Partner A              1           1           1           1           2
Partner B              0           1           1           1           1

问题是,我不知道我将拥有多少产品,因此列号需要根据products表中的行动态更改.

这个非常好的答案似乎不适用于mysql:T-SQL Pivot？可以从行值创建表列

Answer 1

不幸的是,MySQL没有PIVOT基本上你想要做的功能.因此,您需要使用带有CASE语句的聚合函数:

select pt.partner_name,
  count(case when pd.product_name = 'Product A' THEN 1 END) ProductA,
  count(case when pd.product_name = 'Product B' THEN 1 END) ProductB,
  count(case when pd.product_name = 'Product C' THEN 1 END) ProductC,
  count(case when pd.product_name = 'Product D' THEN 1 END) ProductD,
  count(case when pd.product_name = 'Product E' THEN 1 END) 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

请参阅SQL Fiddle with Demo

由于您不了解产品,因此您可能希望动态执行此操作.这可以使用预先准备的语句来完成.

动态数据透视表(将行转换为列)

您的代码如下所示:

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;

请参阅SQL Fiddle with Demo

值得注意的GROUP_CONCAT是,默认情况下限制为1024字节.您可以通过在程序持续时间内将其设置得更高来解决此问题,即.SET @@group_concat_max_len = 32000;