在 MySQL 中使用 COUNT 和 WHERE 连接查询

Question

我正在尝试加入三个查询、2 个表和一个计数，连接本身工作正常：

SELECT * FROM `AccountInfo` NATURAL JOIN `userStats` WHERE `username` = 'x'

我想加入：

SELECT COUNT(following) FROM `followers` WHERE `following` = 'x'

为了获得关注特定用户的关注者数量，但我无法让加入工作。有什么想法吗？

 Account info:

|username|firstname|sirname |address |postcode|
|--------|-------- |--------|--------|--------|
| user1  | first   | last   |road A  |  1234  |
| user2  | name2   | last2  |road b  |  4567  |
|        |         |        |        |        |
|        |         |        |        |        |


  userStats

    |username|followers|following|lastseen |reputation|
    |--------|-------- |-------- |-------- |--------  |
    |  user1 |         |         |DD:MM:YY |  120     |
    |  user2 |         |         | DD:MM:YY|  130    |
    |        |         |         |         |          |
    |        |         |         |         |          |


followers

        |follower|following|
        |--------|-------- |
        |  user1 | user 2  | 
        |  user2 | user 1  | 
        |  user1 | user 3  | 
        |        |         | 

自然加入成功附加了 accountInfo 和 userStats 但我还想计算用户拥有的关注者数量，即用户名出现在“关注”列中的次数，以及他关注的用户数量，即用户名出现在“关注者”中的次数。 （虽然我没有问后半部分）

对于示例数据，它将显示：关注 = 2 和关注者 = 1

我不介意数据是否出现在连接表末尾的单独列中，但理想情况下会代替 userStats 表中的以下/关注者数据

Answer 1

尚不清楚您的输入和期望的输出，但假设我们有以下谓词（参数化语句）：

// user `username` named `firstname` `surname` lives at ...
// == `AccountInfo`(`username`,`firstname`,`surname`,...)

// user `username` was last seen at `lastseen` and ...
// == `userStats`(`username`,`lastseen`,...)

// user `follower` follows user `following`
// == `followers`(`follower`,`following`)

（我的userStats 没有followers 和following 计数列。）

在每个谓词（参数化语句）之后，我为它写了一个简写。每个速记就像SQL CREATE TABLE。一个表（基础或查询结果）包含从其谓词构成一个真实命题（陈述）的行。

（我的回答继续在您的问题中使用NATURAL JOIN。）

满足两个基表谓词 AND 的行表是它们表的 NATURAL JOIN：

//     user `username` named `firstname` `surname` lives at ...
// AND user `username` was last seen at `lastseen` and ...
// ==   `AccountInfo`(`username`,`firstname`,`surname`,...)
// AND `userStats`(`username`,`lastseen`,...)
SELECT * FROM `AccountInfo` NATURAL JOIN `userStats`

满足基表谓词和条件AND的行是基表WHERE条件：

//     `AccountInfo`(`username`,`firstname`,`surname`,...)
// AND `userStats`(`username`,`lastseen`,...)
// AND `username` = 'x'
SELECT * FROM `AccountInfo` NATURAL JOIN `userStats` WHERE `username` = 'x'

现在每个用户的关注者和关注者都很重要：

// user `username` follows user `following`
// == `followers`(`username`,`following`)
SELECT `follower` AS `username` FROM `followers`

// user `username` follows `followers` users
CREATE TABLE `nfollowing`
SELECT `follower` AS `username`, COUNT(`following`) AS following
FROM followers
GROUP BY `follower`

//    user `username` has follower `follower`
// == user `follower` follows user `username`
// == followers(`follower`,`username`)
SELECT `following` AS `username` FROM `followers`

// user `username` has `followers` followers
CREATE TABLE `nfollowers`
SELECT `following` AS `username`, COUNT(`follower`) AS followers
FROM followers
GROUP BY `following`

我们想要满足这些谓词的 AND 的行，即它们的表的 NATURAL JOIN 中的行：

// user `username` has `followers` followers and follows `following` users
// ==  user `username` has `followers` followers
// AND user `username` follows `following` users
CREATE TABLE `nfollow`
SELECT * FROM `nfollowers` NATURAL JOIN `nfollowing`

现在你的 userStats 是：

SELECT * FROM `userStats` NATURAL JOIN `nfollow`

您想要的查询可能是：

SELECT * FROM
   `AccountInfo` NATURAL JOIN `userStats` NATURAL JOIN `nfollow`
WHERE `username` = 'x'

Answer 2

给定以下数据：

CREATE TABLE `accountInfo` (
  `username` varchar(255) DEFAULT NULL,
  `firstname` varchar(255) DEFAULT NULL,
  `sirname` varchar(64) DEFAULT NULL,
  `address` varchar(64) DEFAULT NULL,
  `postcode` varchar(64) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `userstats` (
  `username` varchar(255) DEFAULT NULL,
  `followers` varchar(255) DEFAULT NULL,
  `following` varchar(64) DEFAULT NULL,
  `lastseen` date DEFAULT NULL,
  `reputation` int(64) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE `followers` (
  `follower` varchar(255) DEFAULT NULL,
  `following` varchar(64) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
INSERT INTO `accountInfo` (`username`, `firstname`, `sirname`, `address`, `postcode`)
VALUES
    ('user1', 'first', 'last', 'road a', '1234'),
    ('user2', 'name2', 'last2', 'road b', '4567'),
    ('user3', 'name3', 'last3', 'road c', '8974');


INSERT INTO `userstats` (`username`, `followers`, `following`, `lastseen`, `reputation`)
VALUES
    ('user1', NULL, NULL, '2015-09-22', 120),
    ('user2', NULL, NULL, '2015-09-21', 130),
    ('user3', NULL, NULL, '2015-09-20', 80);

INSERT INTO `followers` (`follower`, `following`)VALUES
('user1', 'user2'),
('user2', 'user1'),
('user1', 'user3');

这些查询将给出用户的关注者数量或关注用户的数量：

select a.*, u.*, count(*) as number_following
from accountinfo a
    left join userstats u on(a.username = u.username)
    left join followers f1 on (u.username = f1.follower)
where 1
group by f1.follower;

select a.*, u.*, count(*) as number_of_followers
from accountinfo a
    left join userstats u on(a.username = u.username)
    left join followers f1 on (u.username = f1.following)
where 1
group by f1.following;

（2015 年 9 月 26 日更新） 您可以在左连接中使用子查询与 group by 来提取计数，例如：

    select a.*, u.*, f1.*, f2.*
   from accountinfo a
        left join userstats u on(a.username = u.username)
        left join (select f.follower as username, count(*) as following_count from followers f group by f.follower) f1 on (u.username = f1.username)
        left join (select f.following as username, count(*) as followers_count from followers f group by f.following) f2 on (u.username = f2.username)
    where 1;