sql_test

Запрос: 

Актуально только для PostgreSQL, т.к. оконные функции есть не везде.

SELECT t.* FROM 
  (
    SELECT 
      news.*, 
      row_number() OVER (PARTITION BY source ORDER BY source, CASE n_type WHEN 'realty' THEN 0 WHEN 'sport' THEN 1 ELSE 2 END, created_at DESC) as rnum 
    FROM news
  ) as t 
WHERE t.rnum <= 5 OR t.n_type NOT IN ('sport', 'realty') ORDER BY t.source, t.rnum;

Схема:

CREATE TABLE news
(
  "id" SERIAL,
  "title" "text",
  "source" "text",
  "n_type" "text",
  "created_at" "timestamp",
  CONSTRAINT "news_pkey" PRIMARY KEY ("id")
);

Данные:

INSERT INTO news (title, source, n_type, created_at) VALUES('news0', '0', 'realty', '2017-03-30 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news1', '0', 'realty', '2017-03-31 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news2', '0', 'sport', '2017-04-01 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news3', '0', 'sport', '2017-04-02 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news4', '0', 'sport', '2017-04-03 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news5', '0', 'sport', '2017-04-04 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news6', '0', 'sport', '2017-04-05 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news7', '0', 'other', '2017-04-06 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news8', '0', 'other', '2017-04-07 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news9', '0', 'other', '2017-04-08 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news10', '1', 'realty', '2017-04-09 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news11', '1', 'realty', '2017-04-10 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news12', '1', 'realty', '2017-04-11 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news13', '1', 'realty', '2017-04-12 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news14', '1', 'realty', '2017-04-13 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news15', '1', 'realty', '2017-04-14 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news16', '1', 'sport', '2017-04-15 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news17', '1', 'other', '2017-04-16 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news18', '1', 'other', '2017-04-17 10:40:00 +0000');
INSERT INTO news (title, source, n_type, created_at) VALUES('news19', '1', 'other', '2017-04-18 10:40:00 +0000');