2

我有三个表的简单文章应用程序：

article 
id, title,  body,   user_id

comment
id, article_id, user_id,    body

user
id, username

在登录页面上，我想显示最新的文章标题以及文章的作者姓名和评论总数。主要问题是如何获得文章的评论总数，我没有做对。我应该得到以下输出：

title           username    total_comments
article 2       user2           0
article 1       user1           2

在我的实际应用程序中，我在文章表中添加了一个列，用于记录文章的评论总数。当向系统添加新评论时，此列会更新。这样做的问题是在添加新评论时文章表被锁定。在我的应用程序中，每分钟都会添加很多评论。所以我试图通过重写 SQL 查询来避免锁定文章表。

以下是一些测试数据：

CREATE TABLE `article` (
`id` INT NULL AUTO_INCREMENT PRIMARY KEY ,
`title` VARCHAR( 100 ) NULL ,
`body` LONGTEXT NULL ,
`user_id` INT NULL
) ENGINE = MYISAM ;


CREATE TABLE `comment` (
`id` INT NULL AUTO_INCREMENT PRIMARY KEY ,
`article_id` INT NULL ,
`user_id` INT NULL ,
`body` LONGTEXT NULL
) ENGINE = MYISAM ;

CREATE TABLE `user` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`username` VARCHAR( 50 ) NOT NULL
) ENGINE = MYISAM ;


INSERT INTO `test`.`user` (
`id` ,
`username`
)
VALUES (
NULL , 'user1'
), (
NULL , 'user2'
);


INSERT INTO `test`.`article` (
`id` ,
`title` ,
`body` ,
`user_id`
)
VALUES (
NULL , 'article 1', 'body article 1', '1'
), (
NULL , 'article 2', 'body article 2', '2'
);

INSERT INTO `test`.`comment` (
`id` ,
`article_id` ,
`user_id` ,
`body`
)
VALUES (
NULL , '1', '1', 'body comment to article 1'
), (
NULL , '1', '1', 'body comment to article 1'
);
4

4 回答 4

3 
SELECT a.title AS title, u.username AS username, count(c.id) as total_comments FROM articles a
   LEFT JOIN comments c ON c.article_id = a.id
   LEFT JOIN users u ON a.user_id = u.id
GROUP BY a.id
于 2011-10-10T06:36:23.207 回答
3

像这样的东西怎么样，现在无法测试它，所以它可能有错误。

SELECT a.title, u.username, COUNT(*) total_comments
FROM article a
  JOIN user u ON (u.id=a.user_id)
  LEFT OUTER JOIN comment c ON (c.article_id=a.id)
GROUP BY a.title, u.username
于 2011-10-10T06:37:23.437 回答
1

如果你试试这个，它会回答吗？

SELECT a.title AS title, 
             u.username AS username, 
             count(c.id) AS total_comments 
 FROM articles a, comments c, users u
 WHERE  a.user_id =u.id
    And a.id=c.articles_id
    And c.user_id = u.id

或者使用内部语法

SELECT a.title AS title, 
             u.username AS username, 
             count(c.id) AS total_comments 
 FROM articles a 
 INNER JOIN comments c ON c.article_id = a.id 
                                          AND c.user_id=u.id
 INNER JOIN users u ON a.user_id = u.id 
 GROUP BY a.id
于 2011-10-10T06:41:33.080 回答
1 
SELECT article.title AS title, COUNT(  'comment.id' ) AS total_comment, user.username AS username
FROM article
JOIN COMMENT ON comment.article_id = article.id
JOIN user ON user.id = article.user_id
GROUP BY article.id
于 2011-10-10T06:44:40.233 回答