SQL GROUP_CONCAT split in different columns - mysql

Question

I searched a lot, but didn't find a proper solution to my problem.
What do I want to do?
I have 2 tables in MySQL:
- Country
- Currency
(I join them together via CountryCurrency --> due to many to many relationship)
See this for a working example: http://sqlfiddle.com/#!2/317d3/8/0
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).
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
This has the following result:
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
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.

Answer 1

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
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.
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).

Answer 2

Use this query to work out the number of currency columns you'll need:
SELECT MAX(c) FROM
((SELECT count(currency.name) AS c
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) as t)
Then dynamically create and execute prepared statement to generate the result, using Gordon Linoff solution with query result above to in this thread.

Answer 3

Ypu can use dynamic SQL, but you will have to use procedure