How do i query this complex query in T-SQL / Linq?

Question

Currently I have 2 table

[Category] -> PK| CAT_ID -> CAT_PARENT (link to itself | if it's a top parent category then it's 0)
[Posts] -> PK | POST_ID -> FK | CAT_ID | CREATE_DATE

How do I select top 15 ROWS of Posts in every CAT_PARENT which have multiple child category. So the total Posts in CAT_PARENT and all it's child are only 15. we have multiple CAT_PARENT here. so we can return multiple group of post which contain 15 post for every CAT_PARENT and it's child category

The problem here is to have it in one round trip of query to the SQL server since one query can take up to 200 posts so it could be the best if you can write it in 1 T-sQL query / linq query.

I prefer if you can write it in LINQ. But it's also OK to write it in T-SQL.

Thank you very much :)

---

Thanks to Alexander's solution down here, i modified some part and it work well with 186 query and some lazy loaded column for less than 2s (remotely) to my SQL server

ALTER procedure [dbo].[get_topParentPost] (
    @quantity int
)
as
    WITH t AS (
    SELECT ROW_NUMBER() OVER (PARTITION BY top_level.CAT_ID ORDER BY p.CREATE_DATE DESC) AS row_num, 
    top_level.CAT_ID AS top_level_cat_id, child_category.CAT_ID AS category_id, p.POST_ID, p.CREATE_DATE, p.VALIDATE,
    p.CAT_ID, p.DESCRIPTION, p.DRAF_OF, p.END_DATE, p.MOD_DATE, p.ON_HOMEPAGE, p.PUBLISH_DATE, p.[STATE], p.THUMB_ID, p.TITLE, p.[TYPE],
    p.[VIEW]
    FROM 
        (SELECT cat_id, 0 as cat_parent FROM Categories c WHERE CAT_PARRENT = 0) AS top_level
        INNER JOIN Categories AS child_category  
          ON child_category.CAT_PARRENT = top_level.CAT_ID OR child_category.CAT_ID = top_level.CAT_ID
        INNER JOIN Posts p 
          ON child_category.CAT_ID = p.CAT_ID AND p.VALIDATE = 1
    )
    SELECT * FROM t WHERE row_num <= @quantity

I modified some part which helps the query select top 15 according to descending date instead of ID ascending

Answer 1

I'm not sure if there's any way to do this with linq without pulling more information than is necessary, but here is a T-SQL script that will accomplish the task. Just to make sure I understood the requirement, here is my description of the script:

This script selects the top 15 entries from the Posts table whose Category has more than 1 sub-category:

declare @cat_id int
declare MULTIPLE_CHILDREN_CATEGORIES cursor for
(
    select cat_id 
    from categories
    group by cat_parent
    having count(*) > 1
)

open MULTIPLE_CHILDREN_CATEGORIES
fetch next from MULTIPLE_CHILDREN_CATEGORIES into @cat_id

while @@fetch_status = 0
begin
    select top 15 *
    from posts
    where cat_id = @cat_id
    order by create_date desc

    fetch next from MULTIPLE_CHILDREN_CATEGORIES into @cat_id
end

close MULTIPLE_CHILDREN_CATEGORIES
deallocate MULTIPLE_CHILDREN_CATEGORIES

Answer 2

If it's for SQL Server 2005 or later, you can do:

SELECT t.top_level_cat_id, t.category_id, t.post_id, t.post_date 
FROM (
    SELECT ROW_NUMBER() OVER (PARTITION BY top_level.cat_id ORDER BY p.post_id) AS row_num, 
        top_level.cat_id AS top_level_cat_id, 
        child_category.cat_id AS category_id, 
        p.post_id, 
        p.post_date
    FROM 
        Post p
        INNER JOIN Category AS child_category 
        ON child_category.cat_id = p.cat_id  
        INNER JOIN Category AS top_level
        ON top_level.cat_id = child_category.cat_parent
) AS t 
WHERE t.row_num <= 15

Here's T-SQL I used for creating test tables:

CREATE TABLE Category (cat_id INT, cat_parent INT);
--top level
INSERT INTO Category VALUES(1, 0);
INSERT INTO Category VALUES(2, 0);
-- child categories
INSERT INTO Category VALUES(3, 1);
INSERT INTO Category VALUES(4, 1);
INSERT INTO Category VALUES(5, 2);

CREATE TABLE Post(post_id INT, cat_id INT, post_date DATETIME);
INSERT INTO Post VALUES(1, 3, GETDATE());
INSERT INTO Post VALUES(2, 3, GETDATE());
INSERT INTO Post VALUES(3, 3, GETDATE());
INSERT INTO Post VALUES(4, 3, GETDATE());
INSERT INTO Post VALUES(5, 3, GETDATE());
INSERT INTO Post VALUES(6, 3, GETDATE());
INSERT INTO Post VALUES(7, 3, GETDATE());
INSERT INTO Post VALUES(8, 3, GETDATE());
INSERT INTO Post VALUES(9, 3, GETDATE());
INSERT INTO Post VALUES(10, 3, GETDATE());
INSERT INTO Post VALUES(11, 3, GETDATE());
INSERT INTO Post VALUES(12, 3, GETDATE());
INSERT INTO Post VALUES(13, 3, GETDATE());
INSERT INTO Post VALUES(14, 3, GETDATE());
INSERT INTO Post VALUES(15, 3, GETDATE());
-- these records won't appear 
INSERT INTO Post VALUES(16, 3, GETDATE());
INSERT INTO Post VALUES(17, 4, GETDATE());
INSERT INTO Post VALUES(18, 4, GETDATE());

INSERT INTO Post VALUES(19, 5, GETDATE());
INSERT INTO Post VALUES(20, 5, GETDATE());

Answer 3

I imagine something like this for linq2sql:

IEnumerable<IEnumerable<Post>> posts = db.Categorys
                        .Where(p=>p.CAT_PARENT == 0)
                        .Select(p=>p.Categorys
                           .SelectMany(q=>q.Posts)
                           .OrderByDescending(q=>q.CREATE_DATE)
                           .Take(15))