Question

我有一个左连接查询来获取用户喜欢的帖子。如果第二次登录用户访问，第一个用户个人资料将显示第一个用户的喜欢，如果第二个用户（登录用户）像同一个帖子那样也会在帖子上显示文字

用户表

user_id | username

喜欢桌子

like_id | post_id | uid

的MySQL

$SQL = "SELECT * FROM  likes LEFT JOIN users ON users.user_id = likes.uid WHERE likes.uid = 'user1'"

如果我在上面查询的while循环中运行另一个查询，它将起作用

$check_id = row['post_id']; //get post id from 1st loop

if(isset($_SESSION['userid'])){

    $check = "SELECT * FROM  likes WHERE post_id='$check_id' AND uid='LOGED-IN-USER-ID'"

}

然后我可以获得num_rows并添加文字。这项工作非常好但我想知道有没有更好的方法来做到这一点，而不在while循环中运行这么多的查询。有没有办法组合查询或在循环外执行第二个查询。

Answer 1

那＆＃34;安全＆＃34;来自＆＃34;数据一致性的观点＆＃34;，但在查询后的一段时间内查询称为＆＃34; 1 + N＆＃34;并且通常是性能杀手，您可以轻松找到有关SQL 1 + N问题的文档。

解决方案是让SQL服务器在单个查询中为您完成工作，避免与它一起打乒乓球（读取：来回的TCP数据包，查询解析，......）。

假设：

> SELECT * FROM user;
+---------+----------+
| user_id | username |
+---------+----------+
|       1 | root     |
|       2 | user2    |
|       3 | user3    |
+---------+----------+


> SELECT * FROM `like`;
+---------+---------+---------+
| like_id | post_id | user_id |
+---------+---------+---------+
|       1 |       1 |       1 |
|       2 |       2 |       1 |
|       3 |       3 |       1 |
|       4 |       4 |       1 |
|       5 |       2 |       2 |
+---------+---------+---------+

> SELECT * FROM `post`;
+---------+--------+
| post_id | text   |
+---------+--------+
|       1 | post 1 |
|       2 | post 2 |
|       3 | post 3 |
|       4 | post 4 |
+---------+--------+

有多种方式可以请求您想要的内容，但有一种方法可能是：

> SELECT like_id, like.post_id, text,
         (SELECT 1 FROM `like`
           WHERE post_id = post.post_id AND
                 user_id = 2 /* logged in user */) AS I_like_it_too
    FROM `like`
    JOIN post USING (post_id)
   WHERE user_id = 1 /* user id of seen profile  */;

+---------+---------+--------+---------------+
| like_id | post_id | text   | I_like_it_too |
+---------+---------+--------+---------------+
|       1 |       1 | post 1 |          NULL |
|       2 |       2 | post 2 |             1 |
|       3 |       3 | post 3 |          NULL |
|       4 |       4 | post 4 |          NULL |
+---------+---------+--------+---------------+

使用I_like_it_too别名根据需要以不同方式显示帖子。

从性能的角度来看，您需要like.user_id上的索引来限制一小部分上的选定行，依赖子查询只会针对此子集运行，因此这样做了行。

另一种可能性是：

>  SELECT displayed.like_id, displayed.post_id, text, my_likes.like_id is not null AS i_also_like
     FROM `like` AS displayed
     JOIN post USING (post_id)
LEFT JOIN `like` AS my_likes ON
          displayed.post_id = my_likes.post_id AND
          my_likes.user_id = 2 /* logged user */
    WHERE displayed.user_id = 1 /* user id of seen profile  */;

+---------+---------+--------+-------------+
| like_id | post_id | text   | i_also_like |
+---------+---------+--------+-------------+
|       1 |       1 | post 1 |           0 |
|       2 |       2 | post 2 |           1 |
|       3 |       3 | post 3 |           0 |
|       4 |       4 | post 4 |           0 |
+---------+---------+--------+-------------+

Answer 2

你的意思是这样吗？

表SO_LIKES（你＆＃34;喜欢＆＃34;表）

like_id | post_id | uid
      1 |       1 | 1
      2 |       2 | 1
      3 |       1 | 2

表SO_USERS（您＆＃34;用户＆＃34;表）

   user_id | username 
         1 | User1 
         2 | User2

SQL

SELECT * FROM SO_LIKES as t1 LEFT JOIN SO_USERS as t2 ON t1.uid = t2.user_id INNER JOIN SO_LIKES as t3 ON t1.post_id = t3.post_id WHERE t2.user_id = 1 AND t3.uid = 2

SO只需再次在您的查询中调用相同表，并使用用户2的ID

WHERE t2.user_id = 1 AND t3.uid = 2

输出看起来像这样

like_id | post_id | uid | user_id | username | like_id | post_id | uid 
      1 |       1 |   1 |       1 |    User1 |       3 |       1 | 2

所以你得到了POST_id 1这两个用户都喜欢

在mysql循环中运行另一个查询是否安全？

2 个答案: