2

I've asked the same question on stackoverlow, but got no answer, so that's why I'm asking it here (I've read that simple sql questions should be asked at SO, but I have no choice).

Suppose I have 3 tables: posts, post_categories and categories. I'm implementing some kind of page with filters where I can filter posts by categories. User can select multiple categories. When he selects more than one, they should be summed. I can't get it working. What I have now is simple IN() SQL clause:

SELECT posts.id, post_categories.category_id FROM posts 
JOIN post_categories ON posts.id = post_categories.post_id 
WHERE post_categories.category_id IN (1,2,3) LIMIT 10;

But it is not matching all ids, it is OR and I need AND. What I need here is to find all posts that have categories with id=1 AND id=2 AND id=3, but instead this query returns posts that have at least one category from the IN() list.

3

The IN() operator is equivalent to a series of OR, which doesn't help you at all here.

Instead, I would build the query in a manner like this.

SELECT posts.id
FROM posts 
WHERE posts.id IN (SELECT post_id FROM post_categories WHERE category_id=1) AND
      posts.id IN (SELECT post_id FROM post_categories WHERE category_id=2) AND
      posts.id IN (SELECT post_id FROM post_categories WHERE category_id=3)

Admittedly, not a pretty construct, but I wrote it for readability. The following query might perform better:

SELECT posts.id
FROM posts
-- include:
WHERE posts.id IN (
        SELECT post_id
        FROM post_categories
        WHERE category_id IN (1, 2, 3)
        GROUP BY post_id
        HAVING COUNT(*)=3)
-- exclude:
    AND posts.id NOT IN (
        SELECT post_id
        FROM post_categories
        WHERE category_id NOT IN (1, 2, 3))

The second query assumes that the primary key of post_categories is (post_id, category_id). Since you haven't specified which rdbms you're running, you may have to tweak my code a bit to make it run.

Edit: The -- exclude: part eliminates posts that have any other category_id than 1, 2 or 3. You may want to skip this part depending on if you want to return a) all posts that have categories 1, 2 and 3 or b) all posts that have exactly categories 1, 2 and 3 and no other categories.

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5

IN is like writing = ANY, e.g.

regress=> SELECT 1 = ANY (ARRAY[1, 2, 3]);
 ?column? 
----------
 t
(1 row)

A simplistic interpretation of your question would be to say that you're asking for = ALL, but that rarely makes sense:

regress=> SELECT 1 = ALL (ARRAY[1, 2, 3]);
 ?column? 
----------
 f
(1 row)

What you really want is to find all posts that have a corresponding post_categories entry for each of a listed set of category_id.

As is usual in relational databases, as soon as you clearly frame the question, the solution starts writing its self.

You could do a multiple join, like Daniel suggests, but that's a pain because it requires dynamic SQL. Or you could use relational set operations.

Instead, I'd possibly use PostgreSQL's array features. This would be easier if you'd provided sample data, but I think you want something like the following, which finds each category that matches the post then does a check to see if all the categories match (the HAVING clause):

SELECT
   posts.id
FROM posts 
INNER JOIN post_categories ON posts.id = post_categories.post_id 
WHERE post_categories.category_id IN (1,2,3)
GROUP BY posts.id
HAVING array_agg(post_categories.category_id) @> ARRAY[1,2,3]
LIMIT 10

Untested, since you didn't provide sample data in CREATE TABLE and INSERTs form, but that's the general idea. @> means "array-contains"; I suspect it'll be more efficient than using array_agg(category_id ORDER BY category_id) and an equality test against a sorted ARRAY literal, because you avoid the sort on aggregation.

You could leave out the WHERE clause and this would still work, but it might be quite slow, as it'd fail to filter out posts in which none of the categories matched before doing aggregation.

BTW, LIMIT without ORDER BY is usually a bug. The database can return any set of 10 matching results it feels like, in any order.

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4

Most importantly, this is a special case of relational division. Once you know the name of the beast you'll find plenty of query techniques. Like the arsenal we assembled on SO:

Building on this test case (which you should have provided):

CREATE TABLE post (
   post_id serial PRIMARY KEY
 , post text NOT NULL
);

CREATE TABLE category (
   category_id serial PRIMARY KEY
 , category text NOT NULL
);

CREATE TABLE post_category (
   post_id int REFERENCES post
 , category_id int REFERENCES category
 , PRIMARY KEY (post_id, category_id)
);

Since I have to provide my own test case I am using proper table and column names. I prefer singular terms for table names where each row represents a single entity. And I avoid the unhelpful "id" as column name.

The special requirement in your case is to make it work with a dynamic set of categories, I suggest two nested EXISTS anti-semi-joins. Should be the fastest and IMO also most elegant way.

This is the task how I understand it after reading the question a couple of times:

"Find all posts that have all of the given categories attached."
Which can be expressed in its inverted form:
"Find all posts where none of the given categories is missing."

Short form with given set of category_id:

SELECT p.*                                   -- "Find all posts ..."
FROM   post p
WHERE  NOT EXISTS (                          -- "where none of the ..."
   SELECT 1
   FROM  (VALUES (1),(2),(3)) c(category_id) -- "given categories ..."
   WHERE  NOT EXISTS (                       -- "is missing"
      SELECT 1
      FROM   post_category pc
      WHERE  pc.post_id = p.post_id
      AND    pc.category_id = c.category_id
      )
   );

Or, retrieving IDs from the category table first:

-- posts with red & green
SELECT p.*
FROM   post p
WHERE  NOT EXISTS (
   SELECT 1
   FROM   category c
   WHERE  c.category IN ('red', 'green') -- retrieve IDs from cat table
   AND    NOT EXISTS (
      SELECT 1
      FROM   post_category pc
      WHERE  pc.post_id = p.post_id
      AND    pc.category_id = c.category_id
      )
   );

SQL Fiddle demonstrating all.

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3

SCOPES can help your out for this.....for example:-

in post

 has_many :post_categories
 has_many :categories
scope :recent_posts, where('created_at < ?', 1.days.ago).order('created_at ASC').limit(10)
scope :recent_posts_with_good_categories, includes(:post_categories).where("post_categories.name=?", "GOOD").merge(Post.recent_post)
scope :recent_posts_with_bad_categories, includes(:post_categories).where("post_categories.name=?", "BAD").merge(Post.recent_post)
scope :recent_posts_with_good_bad__evil_categories, includes(:post_categories).where("post_categories.name in ?", ["GOOD","BAD","EVIL"]).merge(Post.recent_post)


####similarly you can add multiple scopes in other models using includes and merge to get summed results

in post_category

 belongs-to :post
 has_many :categories

similar scopes can come here as well

in category

     belongs-to :post
     belongs-to :post_category
###similar scopes can come here as well

the same can be achieved using class methods and chain them the way u want:-

t=params[:categories_id]

    def self.find_users_with_categories(t)
      ##here you can query with all associated models as they are included
      Post.all(:joins => :post_categories,:categories, :select => "posts.*,categories.name",:group => "posts.id").where("categories.id IN (?)",t)           
      end
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  • IN is still OR behavior and a I need AND — I need to query all posts which only have all of the categories that user chooses. – Dima Knivets Aug 21 '14 at 11:16

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