MySQL 动态枢轴

问题描述

我有一张像这样的产品零件表:

I have a table of product parts like this:

零件

part_id      part_type      product_id
--------------------------------------
1            A              1
2            B              1
3            A              2
4            B              2
5            A              3
6            B              3

而且我想要一个返回这样的表的查询:

and I want a query that will return a table like this:

product_id      part_A_id      part_B_id
----------------------------------------
1               1              2
2               3              4
3               5              6

在其实际实施中将有数百万个产品部件

In its actual implementation there will be millions of product parts

推荐答案

不幸的是，MySQL 没有 PIVOT 函数，但您可以使用聚合函数和 CASE 对其进行建模代码> 语句.对于动态版本，您将需要使用准备好的语句:

Unfortunately, MySQL does not have a PIVOT function but you can model it using an aggregate function and a CASE statement. For a dynamic version, you will need to use prepared statements:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'max(case when part_type = ''',
      part_type,
      ''' then part_id end) AS part_',
      part_type, '_id'
    )
  ) INTO @sql
FROM
  parts;
SET @sql = CONCAT('SELECT product_id, ', @sql, ' 
                  FROM parts 
                   GROUP BY product_id');

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

参见SQL Fiddle with Demo

如果您只有几列，那么您可以使用静态版本:

If you had only a few columns, then you can use a Static version:

select product_id,
  max(case when part_type ='A' then part_id end) as Part_A_Id,
  max(case when part_type ='B' then part_id end) as Part_B_Id
from parts
group by product_id

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