Merge two columns based on a table value - postgresql

Question

I'm trying to merge french strings (language ID 1) into one column. So far, I'm able to get french strings in table1.title and table2.translated_topic, but am not sure how to concatenate them.
Ver: Postgres 9.6.0
Source table schemas:
Table 1: knowledgebase_topics
id | title | language_id |
------------------------------------
64 | The Topic | 91 |
65 | The Topic 2 | 91 |
62 | Le fav sujet | 1 |
63 | Le fav sujet 2 | 1 |
61 | le bonjour | 1 |
Table 2: knowledgebase_topics_translations
id | translated_topic| knowledgebase_topic_id | language_id |
-------------------------------------------------------------
| Le sujet | 64 | 1 |
| Le sujet 2 | 65 | 1 |
| Fav The Topic | 62 | 91 |
| Fav The Topic 2 | 63 | 91 |
Given the following Query:
SELECT title, translated_topic, "kbt".language_id, "kbtt".language_id
FROM knowledgebase_topics as "kbt"
LEFT JOIN knowledgebase_topics_translations as "kbtt" on ("kbtt".knowledgebase_topic_id = "kbt".id)
INNER JOIN knowledgebase_topics_organizations as "kbto" on ("kbto".knowledgebase_topic_id = "kbt".id)
WHERE "kbto"."organization_id" = 1
AND to_tsvector("kbt".title) ## to_tsquery('le')
OR to_tsvector("kbtt".translated_topic) ## to_tsquery('le')
AND "kbt".language_id = 1
OR "kbtt".language_id = 1;
I get the following results:
title | translated_topic | language_id | language_id
----------------+------------------+-------------+-------------
The Topic | Le sujet | 91 | 1
The Topic 2 | Le sujet 2 | 91 | 1
Le fav sujet | Fav The Topic | 1 | 91
Le fav sujet 2 | Fav The Topic 2 | 1 | 91
le bonjour | | 1 |
Desired results: table1.title and table2.translated_topics have been merged based on language_id == 1. Both tables have a language ID column.
title | language_id
----------------+--------------
Le sujet | 1
Le sujet 2 | 1
Le fav sujet | 1
Le fav sujet 2 | 1
le bonjour | 1
How can I do this?
Note: I do not simply want to check lang IDs = 1, such as
and "kbt".language_id = 1 AND (instead of OR) "kbtt".language_id = 1;
Because this results in 2 missing records from table 2 of language ID 1:
title | translated_topic | language_id | language_id
----------------+------------------+-------------+-------------
Le fav sujet | Fav The Topic | 1 | 91
Le fav sujet 2 | Fav The Topic 2 | 1 | 91
le bonjour | | 1 |
So, I've got it working... but is this performant?
SELECT title, "kbt".language_id
FROM knowledgebase_topics as "kbt"
INNER JOIN knowledgebase_topics_organizations as "kbto" on ("kbto".knowledgebase_topic_id = "kbt".id)
WHERE "kbto"."organization_id" = 1
AND to_tsvector("kbt".title) ## to_tsquery('le')
AND "kbt".language_id = 1
UNION ALL
SELECT translated_topic, "kbtt".language_id
FROM knowledgebase_topics_translations as "kbtt"
INNER JOIN knowledgebase_topics_organizations as "kbto" on ("kbto".knowledgebase_topic_id = "kbtt".id)
WHERE "kbto"."organization_id" = 1
AND to_tsvector("kbtt".translated_topic) ## to_tsquery('le')
AND "kbtt".language_id = 1;
Gives output:
title | language_id
----------------+-------------
le bonjour | 1
Le fav sujet | 1
Le fav sujet 2 | 1
Le sujet | 1
Le sujet 2 | 1
(5 rows)

Answer 1

Setting up an environment to answer the question
First, observe how we best describe the problem with concise DDL. Preferably in the future, you'll learn how to write questions like this..
CREATE TEMPORARY TABLE knowledgebase_topics AS
SELECT * FROM ( VALUES
(64,'The Topic',91),
(65,'The Topic 2',91),
(62,'Le fav sujet',1),
(63,'Le fav sujet 2',1),
(61,'le bonjour',1)
) AS t(knowledgebase_topic_id, title, language_id);
CREATE TEMPORARY TABLE knowledgebase_topics_translations AS
SELECT * FROM ( VALUES
('Le sujet' ,64,1 ),
('Le sujet 2' ,65,1 ),
('Fav The Topic' ,62,91 ),
('Fav The Topic 2',63,91 )
) AS t(translated_topic, knowledgebase_topic_id, language_id);
Then you need only tell us what you want and we can get a working environment up easily and answer your question. No English required! Easier on both of us.
The solution
Here we use a UNION ALL we wrap that in a SELECT so we can sort by id, and easily change in one place the language that you're looking for.
SELECT title, language_id
FROM (
SELECT knowledgebase_topic_id, title, language_id
FROM knowledgebase_topics
UNION ALL
SELECT knowledgebase_topic_id, translated_topic, language_id
FROM knowledgebase_topics_translations
) AS t(id, title, language_id)
WHERE language_id = 1
ORDER BY id;
Output
title │ language_id
────────────────┼─────────────
le bonjour │ 1
Le fav sujet │ 1
Le fav sujet 2 │ 1
Le sujet │ 1
Le sujet 2 │ 1
(5 rows)