Select a column by another table's value in PostgreSQL - postgresql

Question

I have Table users:
user_id | lang_id
--------+---------
12345 | en
54321 | ru
77777 | uz
and Table texts:
text_id | en | ru | uz
--------+--------+---------+-------
hi | Hello! | Привет! | Salom!
bye | Bye! | Пока! | Xayr!
I have two informations:
user_id = 12345
text_id = 'hi'
and I'm trying this query, to get a text for user's chosen language:
SELECT (SELECT lang_id FROM users WHERE user_id = 12345) FROM texts
WHERE text_id = 'hi'
and getting this:
lang_id
------
en
I should get the text "Hello!"
I'm kinda newbie in PostgreSQL would be good if you help me to solve this :)

Answer 1

While I 100% recommend changing your schema to something properly normalized like Jorge Campos suggests up in the comments, you can use some hard coding in a CASE statement to get at your texts.
SELECT
CASE
WHEN users.lang_id = 'en' THEN texts.en
WHEN users.lang_id = 'ru' THEN texts.ru
WHEN users.lang_id = 'uz' THEN texts.uz
END as user_language_text
FROM
users, texts
WHERE
user_id = 12345
AND text_id = 'hi';
There are some major downsides here though:
That CASE statement is costly from a CPU perspective
To determine which column in texts from which you retrieve your data you have to hard code the possibly language values. Meaning every time you add a new language not only do you have to add a new column to your table (major anti-pattern by itself) but you also have to tweak ALL of your SQL to accommodate.
You must cross join (or subquery without correlation) to derive the relationship between your texts and the user's lang_id. If lang_id were a column in your text table you would join there and just pick up values in your texts table that correspond to the user's lang_id.
Again. I would highly highly encourage you to rethink your schema since this has you headed toward a nightmare that won't scale, will cause you to constantly edit your schema, and hard code values in your SQL.

Answer 2

Well the simplest way is to cast row to json and then access field dynamically. It is very similar to your original query:
select row_to_json(t.*)->(select lang_id
from users
where user_id = 12345)
from texts t
where text_id = 'hi'

Answer 3

In my opinion it's incorrect approach to develop localization. You can make table texts
with parameters text_id, lang_id and text. And it can help to develop more flexible
In your case you can do it
SELECT
case
when u.user_id = 'en' then t.en
when u.user_id = 'ru' then t.ru
when u.user_id = 'uz' then t.uz
else t.en
end as text
FROM texts as t
left join lateral (
SELECT lang_id
FROM users
WHERE user_id = 12345
) as u on true
WHERE text_id = 'hi'