如何在T-SQL/Linq中查詢這個複雜的查詢？

Question

目前我有2臺

[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 

如何在其中有多個孩子每類選擇CAT_PARENT帖子的前15行。因此，CAT_PARENT中的所有帖子及其所有子帖子都只有15個。我們在此處有多個CAT_PARENT。所以我們可以返回包含每個CAT_PARENT 15個帖子的多個帖子組以及它的子類別

這裏的問題是將它一次往返查詢到SQL服務器，因爲一個查詢最多可能需要200個帖子如果你可以在1個T-sql查詢/ linq查詢中編寫它，它可能是最好的。

我更喜歡，如果你可以寫在LINQ。但也可以在T-SQL中編寫它。

非常感謝你:)

---

由於亞歷山大的解決方案到這裏，我修改了一些部分，並將其與186查詢和一些延遲加載列好工作了不到2秒（遠程）到我的SQL服務器

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 

我修改了一些部分，這有助於查詢按降序日期，而不是ID上升

Answer 1

如果：

這個腳本從帖子表，其類別有超過1子類別選擇前15項它是SQL Server 2005或更高版本，可以這樣做：

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 

這裏的T-SQL我用來創建測試表：

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

我不知道選擇前15名，如果有任何的方式來做到THI與LINQ沒有牽扯更多的信息，但這裏是一個T-SQL腳本，將完成任務。只是爲了確保我瞭解的要求，這裏是我的腳本的描述：

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

我想是這樣的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))