SQL GROUP_CONCAT分为不同的列 [英] SQL GROUP_CONCAT split in different columns

问题描述

我进行了很多搜索，但没有找到解决问题的合适方法.

I searched a lot, but didn't find a proper solution to my problem.

我想做什么?

我在MySQL中有2个表: - 国家 - 货币 (由于多对多的关系，我通过CountryCurrency将他们加入了我们的行列)

I have 2 tables in MySQL: - Country - Currency (I join them together via CountryCurrency --> due to many to many relationship)

请参见以下示例: http://sqlfiddle.com/#!2 /317d3/8/0

我想使用联接将两个表链接在一起，但是我想仅显示每个国家/地区的一行(某些国家/地区使用多种货币，所以这是第一个问题).

I want to link both tables together using a join, but I want to show just one row per country (some countries have multiple currencies, so that was the first problem).

我找到了group_concat函数:

I found the group_concat function:

SELECT country.Name, country.ISOCode_2, group_concat(currency.name) AS currency
FROM country
INNER JOIN countryCurrency ON country.country_id = countryCurrency.country_id
INNER JOIN currency ON currency.currency_id = countryCurrency.currency_id
GROUP BY country.name

结果如下:

NAME            ISOCODE_2   CURRENCY

Afghanistan AF          Afghani
Åland Islands   AX          Euro
Albania         AL          Lek
Algeria         DZ          Algerian Dinar
American Samoa  AS          US Dollar,Kwanza,East Caribbean Dollar

但是我现在想要的是将货币分成不同的列(货币1，货币2，...).我已经尝试过MAKE_SET()之类的功能，但这不起作用.

But what I want now is to split the currencies in different columns (currency 1, currency 2, ...). I already tried functions like MAKE_SET() but this doesn't work.

推荐答案

您可以使用substring_index()进行此操作.以下查询将您的查询用作子查询，然后应用此逻辑:

You can do this with substring_index(). The following query uses yours as a subquery and then applies this logic:

select Name, ISOCode_2,
       substring_index(currencies, ',', 1) as Currency1,
       (case when numc >= 2 then substring_index(substring_index(currencies, ',', 2), ',', -1) end) as Currency2,
       (case when numc >= 3 then substring_index(substring_index(currencies, ',', 3), ',', -1) end)  as Currency3,
       (case when numc >= 4 then substring_index(substring_index(currencies, ',', 4), ',', -1) end)  as Currency4,
       (case when numc >= 5 then substring_index(substring_index(currencies, ',', 5), ',', -1) end)  as Currency5,
       (case when numc >= 6 then substring_index(substring_index(currencies, ',', 6), ',', -1) end)  as Currency6,
       (case when numc >= 7 then substring_index(substring_index(currencies, ',', 7), ',', -1) end)  as Currency7,
       (case when numc >= 8 then substring_index(substring_index(currencies, ',', 8), ',', -1) end)  as Currency8
from (SELECT country.Name, country.ISOCode_2, group_concat(currency.name) AS currencies,
             count(*) as numc
      FROM country
      INNER JOIN countryCurrency ON country.country_id = countryCurrency.country_id
      INNER JOIN currency ON currency.currency_id = countryCurrency.currency_id
      GROUP BY country.name
     ) t

表达式substring_index(currencies, ',' 2)将货币列表以第二个货币为准.对于American Somoa，这将是'US Dollar,Kwanza'.下次以-1作为参数的调用将使用列表的最后一个元素，即'Kwanza'，这是currencies的第二个元素.

The expression substring_index(currencies, ',' 2) takes the list in currencies up to the second one. For American Somoa, that would be 'US Dollar,Kwanza'. The next call with -1 as the argument takes the last element of the list, which would be 'Kwanza', which is the second element of currencies.

还请注意，SQL查询返回一组定义明确的列.查询不能具有可变的列数(除非您通过prepare语句使用动态SQL).

Also note that SQL queries return a well-defined set of columns. A query cannot have a variable number of columns (unless you are using dynamic SQL through a prepare statement).

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