How can I query this complex query in T-SQL / Linq?

Question

I currently have 2 tables

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

How to select the top 15 rows of columns in each CAT_PARENT that have multiple child categories. Thus, the total number of messages in CAT_PARENT and this entire child is only 15. We have several CAT_PARENTs here. therefore, we can return multiple message groups that contain 15 messages for each CAT_PARENT and child category

The problem is to have it in one round of a query to the SQL server, since a single query can receive up to 200 messages, so it may be better if you can write it in 1 T-sQL query / linq query.

I prefer if you can write it in LINQ. But it is also OK to write it in T-SQL.

Many thanks:)

---

Thanks to Alexander’s decision here, I modified the part, and it works well with query 186 and some lazy loaded column in less than 2 s (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 that helps the query select the top 15 according to the descending date instead of the ascending ID

Answer 1

If this is 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 I used T-SQL to create 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 2

, linq, , , T-SQL script, . , , script:

script 15 Posts, 1 :

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 3

I present 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))