问题描述

假设我有三个不同的 MySQL 表:

Lets say I have three different MySQL tables:

表格products:

Table products:

id | name
 1   Product A
 2   Product B

表格partners:

Table partners:

id | name
 1   Partner A
 2   Partner B

表格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

我想要一张表格，行中包含合作伙伴，列中包含产品.到目前为止，我能够得到这样的输出:

I would like to get a table with partners in the rows and products as columns. So far I was able to get an output like this:

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

但我想改为:

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

问题是我无法确定我将拥有多少产品，因此列号需要根据产品表中的行动态更改.

The problem is that I cannot tell how many products I will have so the column number needs to change dynamically depending on the rows in the products table.

这个很好的答案似乎不适用于 mysql:T-SQL 数据透视表?从行值创建表列的可能性

This very good answer does not seem to work with mysql: T-SQL Pivot? Possibility of creating table columns from row values

推荐答案

不幸的是，MySQL 没有 PIVOT 功能，这基本上是您要尝试执行的操作.因此，您需要使用带有 CASE 语句的聚合函数:

Unfortunately MySQL does not have a PIVOT function which is basically what you are trying to do. So you will need to use an aggregate function with a CASE statement:

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

参见 SQL 演示

由于您不知道产品，您可能希望动态执行此操作.这可以使用准备好的语句来完成.

Since you do not know the Products you will probably want to perform this dynamically. This can be done using prepared statements.

使用动态数据透视表(将行转换为列)，您的代码将如下所示:

With dynamic pivot tables (transform rows to columns) your code would look like this:

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;

参见 SQL 演示

可能值得注意的是，GROUP_CONCAT 默认限制为 1024 字节.您可以通过在您的程序期间将其设置得更高来解决此问题，即.SET @@group_concat_max_len = 32000;

It's probably worth noting that GROUP_CONCAT is by default limited to 1024 bytes. You can work around this by setting it higher for the duration of your procedure, ie. SET @@group_concat_max_len = 32000;

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