MySQL Select Where In Many to Many - mysql

Question

I'm having trouble with a SQL query. My schema describes a many to many relationship between articles in the articles table and categories in the categories table - with the intermediate table article_category which has an id, article_id and category_id field.
I want to select all articles which only have the categories with id 1 and 2. Unfortunately this query will also select any articles which have those categories in addition to any others.
For example, this is a sample output from the SQL (with categories shown for descriptive purposes). You can see that while the query selected the article with id of 10, it also selected the article with an id of 11 despite having one extra category.
+-------+------------+
| id | categories |
+-------+------------+
| 10 | 1,2 |
| 11 | 1,2,3 |
+-------+------------+
This is the output that I want to achieve from selecting articles with only categories 1and 2.
+-------+------------+
| id | categories |
+-------+------------+
| 10 | 1,2 |
+-------+------------+
Likewise, this is the output what I want to achieve from selecting articles with only categories 1, 2 and 3.
+-------+------------+
| id | categories |
+-------+------------+
| 11 | 1,2,3 |
+-------+------------+
This is the SQL I have written. What am I missing to achieve the above?
SELECT articles.id
FROM articles
WHERE EXISTS (
SELECT 1
FROM article_category
WHERE articles.id = article_id AND category_id IN (1,2)
GROUP BY article_id
)
Many thanks!

Answer 1

Assuming you want more than just the article's id:
SELECT a.id
,a.other_stuff
FROM articles a
JOIN article_category ac
ON ac.article_id = a.id
GROUP BY a.id
HAVING GROUP_CONCAT(DISTINCT ac.category_id ORDER BY ac.category_id SEPARATOR ',') = '1,2'
If all you want is the article's id then try this:
SELECT article_id
FROM article_category
GROUP BY article_id
HAVING GROUP_CONCAT(DISTINCT category_id ORDER BY category_id SEPARATOR ',') = '1,2'
See it in action at http://sqlfiddle.com/#!2/9d213/4
Should also add that the advantage of this approach is that it can support the checking of any number of categories without having to change the query. Just make '1,2' a string variable and change what gets passed into the query. So, you could just as easily search for articles with categories 1, 2, and 7 by passing a string of '1,2,7'. No additional joins are needed.

Answer 2

You can left join category_id on category.id and then GROUP_CONCAT to get all categories, like you wrote in explanation (1st table) and than using HAVING match with any set you like ( '1,2' from example)
also with that approach you can easily make this query dynamic with php or any other language
SELECT articles.id
FROM articles
WHERE EXISTS (
SELECT GROUP_CONCAT(c.id) AS grp
FROM article_category
LEFT OUTER JOIN categories AS c ON c.id = article_category.category_id
WHERE articles.id = article_id
GROUP BY article_id
HAVING grp = '1,2'
)

Answer 3

Please Use Below Query You can do the Thing by Using Simple Query.
SELECT a.id, a.name
FROM articles a, categories c, articles_categories ac
WHERE
a.id = ac.article_id AND c.id = ac.category_id
AND c.id = 1 OR c.id = 2;
NOTE- If You have Many to many Relationship between two Tables Remove ID from the article_category table and make composite primary key Using article_id and category_id.
Thank you.

Answer 4

Maybe, something like:
select distinct article_id from article_cathegory
where category_id in (1,2)
minus
select distinct article_id from article_cathegory
where category_id not in (1,2)

Answer 5

Looks like simple solution for this could be the following:
SELECT
ac.article_id
, SUM(ac.category_id IN (1, 2)) AS nb_categories
, COUNT(ac.category_id) AS nb_all_categories
FROM
article_categories ac
GROUP BY
ac.article_id
HAVING
nb_categories=2 AND nb_all_categories=2
Here I count how many required categories we have and also count how many categories we have in total. We only need exactly 2 categories, so both required and total should be equal to 2.
This is quite flexible and for adding more categories just change categories list and numbers in HAVING statement.

Answer 6

SELECT articles.id
FROM articles
INNER JOIN articles_category ac ON articles.id = ac.article_id
WHERE articles.id IN (
SELECT ac1.article_id
FROM article_category ac1
WHERE ac1.category_id = 1;
)
AND ac.article_id = 2;
AND articles.id NOT IN (
SELECT ac2.article_id
FROM article_category ac2
WHERE ac2.category_id NOT IN (1, 2)
)
Far from the prettiest one I have written.
Basically, it limits first by ID that have a category id of 1, then it makes sure the records have a category of 2 as well, finally, it makes sure that it does not have any other categories

Answer 7

I like to approach these queries using group by and having. Here is an example:
select ac.article_id
from article_category ac
group by ac.article_id
having sum(case when category_id = 1 then 1 else 0 end) > 0 and
sum(case when category_id = 1 then 2 else 0 end) > 0;
Each condition in the having clause is testing for the presence of one of the categories.
I find that this approach is the most flexible for answering many different types of "set-within-sets" problems.
EDIT:
A slight variation on the above might be easier to generate:
having sum(category_id in (1, 2, 3)) = count(*) and
count(*) = 3
This will work assuming there are no duplicates in the data. You need to update the 3 to be the number of items in the in list.

Answer 8

to help just without changing your query very much, i think the logic has a bug. you dont want articles where a categegory 1,2 exists. You need articles where does not exist categories different than 1 and 2.
thanks & regards

Answer 9

In SQL Server I would do it with an INTERSECT and an EXCEPT. In MySQL, try this:
SELECT DISTINCT article_id
FROM article_category
WHERE category_id=1
AND article_id IN
(SELECT article_id
FROM article_category
WHERE category_id=2)
AND article_id NOT IN
(SELECT article_id
FROM article_category
WHERE category_id NOT IN (1,2))

Answer 10

Use this SQL query.
SELECT articles.id
FROM articles
WHERE articles.id in (
SELECT *
FROM article_category,articles
WHERE article_category.articles.id = articles.article_id AND article_category.category_id IN (1,2)
)