Comments on: Conditional Joins in MySQL

By: Rajitha

Rajitha — Wed, 09 Nov 2011 18:42:58 +0000

Nice !!! Helpful!!!

By: Hazan Ilan

Hazan Ilan — Thu, 24 Feb 2011 07:46:27 +0000

Hi Stefan,
I have fixed it.
Many thanks

By: Stefan

Stefan — Wed, 23 Feb 2011 17:15:53 +0000

Hi, thanks for the example.
Just to remark: the SQL Query has typing errors…

Following the correct code:

SELECT
q.q_id,
q.q_timestamp,
IF(q.q_is_answered, a.q_title, u.q_title) AS q_titleunanswered_questions_title
FROM
questions q
LEFT JOIN answered_questions_title a
ON (q.q_id = a.q_id AND q.q_is_answered = 1)
LEFT JOIN unanswered_questions_title u
ON (q.q_id = u.q_id AND q.q_is_answered = 0);

Greetings

Stefan

By: Diego

Diego — Thu, 27 Jan 2011 19:16:15 +0000

@Andrei, if you do understand that query as you formatted it… my most sincere congratulations: you are a genious. Just in case you don’t know, linefeeds in sql queries do not affect the results. Not using linefeeds, on the other hand, does affect our braincells and our eyes.

By: Hazan Ilan

Hazan Ilan — Mon, 10 Jan 2011 22:25:59 +0000

Andrei,
Use the left join that I mentioned in the post. It will allow you to include more than one column to the result.

By: Hazan Ilan

Hazan Ilan — Mon, 10 Jan 2011 22:22:20 +0000

John,
Your selects statements in the If Clause should return only one result and not three.

By: John 5th

John 5th — Thu, 06 Jan 2011 20:06:29 +0000

@Andrei
I have reformated you query but I guess it’s not correct at all:

SELECT(
CONCAT_WS(‘ ‘, TRIM(journalists.name), TRIM(journalists.surname)) as user_name,
journalists.photo as user_photo,
journalists.posts as user_posts

WHERE
journalists_replay.user_id = jounalists.id

SELECT (
CONCAT_WS(‘ ‘, TRIM(prs.name), TRIM(prs.surname)) as user_name,
prs.photo as user_photo,
prs.posts as user_posts )

FROM
journalists_replay

WHERE
journalists_replay.post_id = ’69′ AND journalists_replay.enabled

ORDER BY
journalists_replay.date DESC;

By: Andrei

Andrei — Sun, 07 Mar 2010 23:31:00 +0000

This case is not a solution for followed query:

SELECT journalists_replay.id AS replay_id, journalists_replay.date AS replay_date, journalists_replay.user_id AS user_id, journalists_replay.user_type AS user_type, journalists_replay.text AS replay_text, IF(journalists_replay.user_type=’journalists’, SELECT(CONCAT_WS(‘ ‘, TRIM(journalists.name), TRIM(journalists.surname)) as user_name, journalists.photo as user_photo, journalists.posts as user_posts WHERE journalists_replay.user_id = jounalists.id, SELECT(CONCAT_WS(‘ ‘, TRIM(prs.name), TRIM(prs.surname)) as user_name, prs.photo as user_photo, prs.posts as user_posts) FROM journalists_replay WHERE journalists_replay.post_id = ’69′ AND journalists_replay.enabled ORDER BY journalists_replay.date DESC;

I can’t find a solution for this.
Best regards.

By: Ben

Ben — Thu, 25 Feb 2010 09:20:20 +0000

Consider using subqueries:

SELECT
q_id,
q_timestamp,
IF(q_is_answered,
SELECT q_title FROM answered_questions WHERE q_id=q.q_id,
SELECT q_title FROM unanswered_questions WHERE q_id=q.q_id) as q_title
FROM
questions q

I believe this is clearer, shorter and will run faster.
(It also assumes that each of the subqueries will return a single value.
The original solution does not handle this situation well either.)