我怎样才能让mysql在标签搜索中获得平均评分？

Question

我想让我的脑袋围绕一个复杂的mysql语句（对我来说很复杂！）。 基本上，我需要返回产品表 中的所有产品的清单，并附带额外的返回值（其各自的星级评定表（评级表），其中 必须按该产品的所有评级的总和计算） 。

SQL语句还必须包括过滤基于 多个“标签”字，例如搜索被链接 （通过product_tags表标签表）中的所有产品，以指定的字的产品的能力建设 的时sql语句。因此，如果我需要检索标签为“红色”和“白色”的产品，则结果会返回产品1和3并获得各自的平均评级。

下面是示例表的一个sql转储。

DROP TABLE IF EXISTS `product_tags`; 
DROP TABLE IF EXISTS `rating`; 
DROP TABLE IF EXISTS `tags`; 
DROP TABLE IF EXISTS `products`; 
CREATE TABLE IF NOT EXISTS `products` (
    `product_id` int(11) NOT NULL AUTO_INCREMENT, 
    `product_name` varchar(255) CHARACTER SET latin1 NOT NULL, 
    `date_added` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, 
    PRIMARY KEY (`product_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=4 ; 

INSERT INTO `products` (`product_id`, `product_name`, `date_added`) VALUES 
(1, 'first item', '2011-05-26 21:56:06'), 
(2, 'second item', '2011-05-26 21:56:06'), 
(3, 'third item', '2011-05-26 21:56:06'); 


CREATE TABLE IF NOT EXISTS `product_tags` (
    `product_id` int(10) unsigned NOT NULL, 
    `tag_id` int(10) unsigned NOT NULL, 
    KEY `product_id` (`product_id`), 
    KEY `tag_id` (`tag_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

INSERT INTO `product_tags` (`product_id`, `tag_id`) VALUES 
(1, 4), 
(1, 1), 
(1, 8), 
(2, 3), 
(2, 9), 
(3, 8), 
(3, 7), 
(1, 6), 
(2, 5), 
(3, 2), 
(3, 10); 

CREATE TABLE IF NOT EXISTS `rating` (
    `product_id` int(11) NOT NULL, 
    `rating` float NOT NULL, 
    KEY `product_id` (`product_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

INSERT INTO `rating` (`product_id`, `rating`) VALUES 
(1, 5), 
(1, 0), 
(2, 3), 
(2, 4.5), 
(1, 2), 
(2, 4); 

CREATE TABLE IF NOT EXISTS `tags` (
    `tag_id` int(10) unsigned NOT NULL AUTO_INCREMENT, 
    `tag_name` varchar(50) NOT NULL, 
    PRIMARY KEY (`tag_id`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=11 ; 

INSERT INTO `tags` (`tag_id`, `tag_name`) VALUES 
(1, 'red'), 
(2, 'green'), 
(3, 'yellow'), 
(4, 'cyan'), 
(5, 'blue'), 
(6, 'pink'), 
(7, 'purple'), 
(8, 'grey'), 
(9, 'black'), 
(10, 'white'); 

ALTER TABLE `product_tags` 
    ADD CONSTRAINT `product_tags_ibfk_2` FOREIGN KEY (`tag_id`) REFERENCES `product_tags` (`tag_id`) ON DELETE CASCADE ON UPDATE CASCADE, 
    ADD CONSTRAINT `product_tags_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `product_tags` (`product_id`) ON DELETE CASCADE ON UPDATE CASCADE; 

ALTER TABLE `rating` 
    ADD CONSTRAINT `rating_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `products` (`product_id`) ON DELETE CASCADE ON UPDATE CASCADE; 

Answer 1

SELECT 
    p.product_id 
    , p.product_name 
    , p.date_added 
    , (SELECT AVG(r.rating) 
     FROM rating r 
     WHERE r.product_id = p.product_id 
    ) 
    AS avg_rating 
FROM 
    products p 
    JOIN 
    product_tags pt 
     ON pt.product_id = p.product_id 
    JOIN 
    tags t 
     ON t.tag_id = pt.tag_id 
WHERE 
    t.tag_name IN ('red','white') 
GROUP BY 
    p.product_id 

---

作为一个侧面说明，最好使用单数的表名。 product，tag，product_tag（如rating这已经是单数），而不是复数你这样做：products等

Answer 2

SELECT `products`.`product_id`, `product_name`, `date_added`, 
     AVG(`rating`) avg_rating, 
     GROUP_CONCAT(`tags`.`tag_name`) all_tags 
    FROM `products` 
    JOIN `product_tags` ON `products`.`product_id` = `product_tags`.`product_id` 
    JOIN `tags` ON `product_tags`.`tag_id` = `tags`.`tag_id` 
    LEFT JOIN `rating` ON `products`.`product_id` = `rating`.`product_id` 
WHERE `tags`.`tag_name` in (?) 
GROUP BY `products`.`product_id` 

Answer 3

select 
    p.product_id 
    , p.product_name 
    , group_concat(distinct tag_name) as tags 
    , ifnull(avg(r.rating),'no rating') as avg_rating 
from products p 
left join rating r on (r.product_id = p.product_id) 
inner join product_tags pt on (pt.product_id = p.product_id) 
inner join tags t on (t.tag_id = pt.tag_id) 
where tag_name in ('red','white') 
group by p.product_id 

结果：

1, 'first item', 'red', '2.33333333333333' 
3, 'third item', 'white', 'no rating'