SQL查询（CASE）中的流控制/条件

Question

I have a CMS that is multi-lingual. There are about 300 words and phrases used throughout the system that change when the user changes the selected language. English is the default language. Other languages may not, at any given point, have been completely translated. I want the English version to be the fall back on a case by case basis where a particular word or phrase has not been translated into the selected language. (stack overflow doesn't allow the table html tag, so I will show two records using an unordered list)

language_id label         translation
en          your_account  Your Account
zh          your_account  您的賬戶

Primary key is on language_id and label

I am using PDO on MariaDB. The part of my query that I am stuck on is the if/case statement, which I know is completely wrong below, but it demonstrates what I wish to accomplish :

SELECT translation_result 
FROM (SELECT translation as translation_result FROM languages WHERE
label='your_account' AND language_id = 'zh')A
CASE  A.translation_result IS NOT NULL ELSE
(SELECT translation as translation_result FROM languages WHERE
label='your_account' AND language_id = 'en')B

Answer 1

Here's what I would do :

SELECT (IFNULL(ZH.translation, EN.translation)) AS translation_result
FROM languages EN 
LEFT JOIN languages ZH ON EN.label=ZH.label AND ZH.language_id = 'zh'
WHERE EN.label='your_account' 
  AND EN.language_id = 'en'

Since english is the master language and always populated, you make your base query on the english records, which always returns something. From what the english returns, you make a LEFT JOIN on the same table, for the same label but with diffrent language. If the JOIN is matching something (you don't have NULL ) you use this value

But I think the query you shown is not a good example as it is not representative of what your end-queries on language table will be in the application. Normally you should make your "master" query on other table(s), and add LEFT JOIN queries on the language table(s) following the user's language You can add a real example and I'll adapt my answer to match it.