MySQL - Left Join with Case select sets NULL value in first CASE - mysql

Question

Scenario
I have a users table that has a column for the users iso_code_2 for their country of residence and nationality, and in another table I have all the countries in different languages, so what I want to do is get the country text for the users residence and nationality. I know the problem is the GROUP BY but I do not know how to solve it.
Tables
/* Users table */
╔══════╦═════════════╦════════════╦═════════════╦═══════════════╗
║ id ║ firstname ║ lastname ║ residence ║ nationality ║
╚══════╩═════════════╩════════════╩═════════════╩═══════════════╝
│ 1 │ Joe │ Doe │ JP │ PH │
├──────┼─────────────┼────────────┼─────────────┼───────────────┤
│ 2 │ Lisa │ Simpson │ US │ AR │
├──────┼─────────────┼────────────┼─────────────┼───────────────┤
│ 3 │ Homer │ Simpson │ JP │ JP │
└──────┴─────────────┴────────────┴─────────────┴───────────────┘
/* Countries table */
╔══════╦═══════════════╦══════════════╦═════════════════════╗
║ id ║ language_id ║ iso_code_2 ║ country ║
╚══════╩═══════════════╩══════════════╩═════════════════════╝
│ 1 │ 1 │ JP │ Japan │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 2 │ 2 │ JP │ 日本 │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 3 │ 1 │ PH │ Philippines │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 4 │ 2 │ PH │ フィリピン │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 5 │ 1 │ US │ United States │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 6 │ 2 │ US │ 米国 │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 7 │ 1 │ AR │ Argentina │
├──────┼───────────────┼──────────────┼─────────────────────┤
│ 8 │ 2 │ AR │ アルゼンチン │
└──────┴───────────────┴──────────────┴─────────────────────┘
/* Expected results */
╔══════╦═════════════╦════════════╦════════════════════════╦═══════════════════════╗
║ id ║ firstname ║ lastname ║ residence_country ║ nationality_country ║
╚══════╩═════════════╩════════════╩════════════════════════╩═══════════════════════╝
│ 1 │ Joe │ Doe │ Japan │ Philippines │
├──────┼─────────────┼────────────┼────────────────────────┼───────────────────────┤
│ 1 │ Lisa │ Simpson │ United States │ Argentina │
├──────┼─────────────┼────────────┼────────────────────────┼───────────────────────┤
│ 1 │ Homer │ Simpson │ Japan │ Japan │
└──────┴─────────────┴────────────┴────────────────────────┴───────────────────────┘
Current Query
SELECT
u.id,
u.firstname,
u.lastname,
CASE c.iso_code_2
WHEN u.nationality THEN c.country
END AS nationality_country,
CASE c.iso_code_2
WHEN u.residence THEN c.country
END AS residence_country
FROM
users AS u
LEFT JOIN
countries AS c ON c.language_id = 1 WHERE c.iso_code_2 IN (u.nationality, u.residence)
GROUP BY u.id
ORDER BY u.created_at DESC
LIMIT 15
Wrong results
╔══════╦═════════════╦════════════╦════════════════════════╦═══════════════════════╗
║ id ║ firstname ║ lastname ║ residence_country ║ nationality_country ║
╚══════╩═════════════╩════════════╩════════════════════════╩═══════════════════════╝
│ 1 │ Joe │ Doe │ NULL │ Philippines │
├──────┼─────────────┼────────────┼────────────────────────┼───────────────────────┤
│ 1 │ Lisa │ Simpson │ NULL │ Argentina │
├──────┼─────────────┼────────────┼────────────────────────┼───────────────────────┤
│ 1 │ Homer │ Simpson │ NULL │ Japan │
└──────┴─────────────┴────────────┴────────────────────────┴───────────────────────┘

Answer 1

You group by is retaining only one row per user in the result. Depending on MySQL's preferences, it will either contain the residence_country or the nationality_country.
You need to select twice from the country table to get your desired results (and it will make the query easier)
SELECT
u.id,
u.firstname,
u.lastname,
cn.country
cr.country
FROM
users AS u
LEFT JOIN countries AS cn ON cn.language_id = 1 WHERE cn.iso_code_2 = u.nationality
LEFT JOIN countries AS cr ON cr.language_id = 1 WHERE cr.iso_code_2 = u.residence