How can I use a join to combine these two queries? - mysql

Question

I have two tables: articles and modifications. I want users to easily be able to revert their article back to it's original state if they realize they shouldn't have modified it. Instead of using an extra query to find the id of the article's newest modification, I would like to use a join. So I want to get the information from the articles table and then join the modifications table to it to return the associated row. This is what I have now:
<?php
$query = "
SELECT
article_id, title, content
FROM articles
WHERE article_id = ".$article_id."
LIMIT 1";
$query_article = $this->db->query($query);
$article = $query_article->row_array();
$query_mod = "
SELECT
modification_id, article_id, title, content, date
FROM modifications
WHERE article_id = ".$article_id."
ORDER BY modification_id DESC
LIMIT 1";
$query_mod = $this->db->query($query);
if($query_mod->num_rows() > 0){
$mod = $query_mod->row_array();
$article_title = $mod['title'];
$article_content = $mod['content'];
} else {
$article_title = $article['title'];
$article_content = $article['content'];
}
How could I combine these two queries into one using a join?
SELECT
a.title, a.content, a.article_id
m.modification_id AS mod_id, m.title AS mod_title, m.content AS mod_content
FROM articles AS a
LEFT JOIN modifications AS m ON (...)
WHERE a.article_id = 1
LIMIT 1

Answer 1

Your skeletal attempt at a query is pretty much correct except for the empty ON clause, which merely needs to identify equality between article_id on the two tables. It is correct to use a LEFT JOIN since you need to return the article regardless of a match in modifications.
SELECT
a.title, a.content, a.article_id,
m.modification_id AS mod_id, m.title AS mod_title, m.content AS mod_content
FROM
articles AS a
LEFT JOIN modifications AS m ON a.article_id = m.article_id
WHERE a.article_id = 1
ORDER BY mod_id DESC LIMIT 1
However, your PHP logic shows that you are conditionally using the title,content from the modifications table if it is present. For that, you may use COALESCE() directly in the SQL to return the first non-null argument, so if the LEFT JOIN has no match, article will be used.
SELECT
-- COALESCE to prefer the modifications value if non-null
COALESCE(m.title,a.title) AS title,
COALESCE(m.content, a.content) AS content,
a.article_id,
m.modification_id AS mod_id
FROM
articles AS a
LEFT JOIN modifications AS m ON a.article_id = m.article_id
WHERE a.article_id = 1
ORDER BY mod_id DESC LIMIT 1
Here's a demonstration: http://sqlfiddle.com/#!2/1085c/1
Because you're only attempting to return details for one article_id, no subqueries are needed. This gets a little more complicated if you want to return the latest for multiple article_id, requiring a subquery join with a MAX() aggregate.
SELECT
COALESCE(m.title,a.title) AS title,
COALESCE(m.content, a.content) AS content,
a.article_id,
m.modification_id AS mod_id
FROM
articles AS a
-- Join articles against a subquery to get the most recent mod_id only
LEFT JOIN (
SELECT article_id, MAX(modification_id) AS mod_id
FROM modifications
GROUP BY article_id
) mm ON mm.article_id = a.article_id
-- and then join that mod_id against the rest of the modifications table
LEFT JOIN modifications m ON mm.mod_id = m.modification_id
WHERE article_id IN (<multiple criteria for article_id>)
http://sqlfiddle.com/#!2/14051/2