如果存在N个特殊行，如何 Select 它们，其余的必须填充常规行，总行数不能超过MySQL中的X行

假设有一个在一个页面上显示10种产品的门户. 期望的目标是在first页面上显示up to 3 randomly chosen "is_featured"个产品，以及其他产品if they exist.If not，只展示10 regular种产品. 查询应该包含限制和偏移量，因为有分页.浏览门户时不能跳过任何行，也不能重复.

第一页上的结果可能类似于此(前3行是"is_Featureed"，任意 Select )，其余行是排序的，在本例中是按Item_id排序的.

item_id | author              | is_featured
129     | Bohumil Peterka     | 1
102     | Aneta Šebková       | 1
150     | Jakub Šustr         | 1
100     | Richard Kovář       | 0
101     | Růžena Staňková     | 0
103     | Lubomír Hladík      | 0
104     | Ing. Dalibor Lang   | 0
105     | Miloš Formánek      | 0
106     | Michal Hlaváček     | 0
107     | Jarmila Seidlová    | 0

然后，第2页将继续使用Item_id 108...

我到目前为止所try 的

(
  SELECT *
  FROM item
  WHERE is_featured = 1
    ORDER BY RAND()
  LIMIT 3 
)
UNION ALL
(
  SELECT *
  FROM item
  ORDER BY item_id
  LIMIT 7 
  OFFSET 0
)

我的查询有多个问题

1. 结果集有时可能返回重复项，我想我可以使用SELECT DISTICT/TEMPORARY TABLE，这将解决这个问题

2. 如果没有"IS_FEATURED"项，结果集将只返回7行，而不是10行

创建表格

CREATE TABLE `item` (
    `item_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
    `author` TEXT NULL DEFAULT NULL,
    `is_featured` TINYINT(3) UNSIGNED NULL DEFAULT '1'
);

样本数据

INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (100, 'Richard Kovář', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (101, 'Růžena Staňková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (102, 'Aneta Šebková', 1);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (103, 'Lubomír Hladík', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (104, 'Ing. Dalibor Lang', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (105, 'Miloš Formánek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (106, 'Michal Hlaváček', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (107, 'Jarmila Seidlová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (108, 'René Sehnal', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (109, 'Jarmila Kvapilová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (110, 'Zdeňka Hanušová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (111, 'Peter Štefek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (112, 'Veronika Pšeničková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (113, 'Ivan Hrabal', 1);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (114, 'Vladimíra Pavlíčková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (115, 'Lenka Dušková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (116, 'Hana Bendová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (117, 'Radim Horváth', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (118, 'Dana Smržová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (119, 'Romana Divišová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (120, 'Ondřej Kropáček', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (121, 'Alena Šebestová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (122, 'Matěj Kurka', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (123, 'Andrea Rambousková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (124, 'Alena Kaňová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (125, 'Vladimír Sládek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (126, 'Šárka Smrčková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (127, 'Jiřina Papežová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (128, 'Radomír Martínek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (129, 'Bohumil Peterka', 1);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (130, 'Bc. Karel Vejvoda', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (131, 'Jiří Hladík', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (132, 'Miluše Holečková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (133, 'Jaromír Mareš', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (134, 'MVDr. Marcela Šafářová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (135, 'Rudolf Duda', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (136, 'Irena Husáková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (137, 'Simona Bednářová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (138, 'MUDr. Peter Landa', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (139, 'Kristýna Hynková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (140, 'Helena Kudrnová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (141, 'Tomáš Hájek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (142, 'Jindřich Ulrich', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (143, 'Vlastimil Sobek', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (144, 'Ivo Lacina', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (145, 'JUDr. Nela Králová', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (146, 'Alena Horká', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (147, 'Dalibor Žižka', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (148, 'Aneta Mráčková', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (149, 'Ondřej Holík', 0);
INSERT INTO `item` (`item_id`, `author`, `is_featured`) VALUES (150, 'Jakub Šustr', 1);