SQL合并两个表-每个键值只获取一行

Question

我正在尝试在我的数据库上运行 SQL 命令。我有两张表，一张用于用户，另一张用于帖子。我想找到每个用户的最后一篇文章。我尝试了以下操作：

SELECT b.`username`, b.`id`, c.`home`, c.`text` FROM `sunlight-users` AS b 
LEFT JOIN `sunlight-posts` AS c ON b.`id` = c.`author`
WHERE b.`group` = 1 OR b.`group`=3 OR b.`group`= 5 OR b.`group` = 10 OR b.`group` = 14 OR b.`group` = 
23 OR b.`group` = 25
ORDER by c.`time` DESC
GROUP BY b.`username`

失败并出现错误“您的语法在第 5 行的 GROUP BY b.username 附近很奇怪”。 我尝试使用 while 循环（在网页上）运行命令，如下所示：

$query = DB::query("SELECT * FROM `"._mysql_prefix."-users` WHERE `group` = 1 OR `group` = 3 OR `group` = 5 OR `group` = 10 OR `group` = 14 OR `group` = 23 OR `group` = 25");

    while($item = DB::row($query)) {
        $post = DB::query("SELECT type,home,xhome,author,time FROM `"._mysql_prefix."-posts` WHERE ".$knihy." AND `author` = ".item['id']." ORDER BY `time` DESC LIMIT 1");
        switch($post['home']){
            case 2:
           //code
                break;

            case 3:
             //code
            break;

            default:
             //code
        }

但是看起来第二个查询被完全忽略了，即使手动将查询输入数据库得到 2 或 3，我也会得到默认值。我知道在循环中运行 50 个查询可能不是一个好主意，那就是为什么我要一次完成所有事情。 这段代码：

SELECT b.`username`, b.`id`, c.`home`, c.`text` FROM `sunlight-users` AS b 
    LEFT JOIN `sunlight-posts` AS c ON b.`id` = c.`author`
    WHERE b.`group` = 1 OR b.`group`=3 OR b.`group`= 5 OR b.`group` = 10 OR b.`group` = 14 OR b.`group` = 
    23 OR b.`group` = 25
    ORDER by c.`time` DESC

结果

+----+----------+------+------+
| id | username | home | text |
+----+----------+------+------+
|  1 | user1    |    2 | .... |
|  1 | user1    |    3 | aaaa |
|  0 | user0    |    4 | .... |
+----+----------+------+------+

顺序正确，我只想要每个用户的上一行。如有任何帮助，我将不胜感激。

Answer 1

您实际上需要过滤，而不是分组。您可以使用相关子查询来做到这一点：

select 
    u.username, 
    u.id, 
    p.home, 
    p.text 
from sunlight_users u
left join sunlight_posts p
    on  u.id = p.author
    and p.time = (select max(p1.time) from sunlight_posts p1 where p1.author = u.id)
where u.group in (1, 3, 5, 10, 14, 23, 25)
order by u.username

在 MySQL 8.0 中，您可以使用窗口函数：

select *
from (
    select 
        u.username, 
        u.id, 
        p.home, 
        p.text,
        row_number() over(partition by u.id order by p.time desc) rn
    from sunlight_users u
    left join sunlight_posts p on  u.id = p.author
    where u.group in (1, 3, 5, 10, 14, 23, 25)
) t
where rn = 1
order by u.username