Question

我需要从这3个表中检索数据

users
+----------+-----------+
| username | last_name |
+----------+-----------+
| name     | last name |
+----------+-----------+
messages
+----+---------+-----------+---------+------+---------------------+
| id | to_user | from_user | message | read | sent                |
+----+---------+-----------+---------+------+---------------------+
| 12 |      12 |        10 | 1       |    0 | 2012-01-10 17:06:14 |
+----+---------+-----------+---------+------+---------------------+
all_messages
+----+--------+--------+
| id | user_1 | user_2 |
+----+--------+--------+
|  1 |     10 |     12 |
+----+--------+--------+

我需要选择所有这些数据，但仅限user_1等于random number的地方并且user_2等于from_user且user_2等于users。id现在如果只有2个表我选择我需要的数据没问题但我不知道如何加入表3，users我的当前查询没有包含users表：

SELECT `all_messages`.`user_1`, `messages`.*
FROM `all_messages`
JOIN `messages` 
ON (`all_messages`.`user_2` = `messages`.`from_user`) 
WHERE `all_messages`.`user_1` = '12'
ORDER BY `messages`.`id` DESC LIMIT 2

编辑感谢您的答案，它完美无缺，

但是，如果我只需要从messages获取最后一条记录，那该怎么办呢？因为现在它获得all_messages.user_2 = messages.from_user的所有记录，而我只需要获得最后一个newest记录

Answer 1

您只需使用第三个表格为您的查询添加另一个“加入”：

SELECT `all_messages`.`user_1`, `messages`.*
FROM `all_messages`
JOIN `messages` 
ON (`all_messages`.`user_2` = `messages`.`from_user`) 
JOIN  `users`
ON (`users`.`id` = `all_messages`.`user_2`)
WHERE `all_messages`.`user_1` = '12'
ORDER BY `messages`.`id` DESC LIMIT 2

Answer 2

您只需添加一个新的联接，如下所示：

SELECT `all_messages`.`user_1`, `messages`.*
FROM `all_messages`
JOIN `messages` ON (`all_messages`.`user_2` = `messages`.`from_user`) 
JOIN users      ON (...write your condition here...)
WHERE `all_messages`.`user_1` = '12'
ORDER BY `messages`.`id` DESC LIMIT 2

Answer 3

SELECT
    users.username,users.last_name,messages.* 
FROM
    (SELECT * FROM all_messages WHERE user_1=12) all_msgs
    INNER JOIN users ON messages.id = users.id
    INNER JOIN messages ON all_msgs.user_2 = messages.from_user
ORDER BY
    messages.id DESC LIMIT 2
;

一起加入3张桌子

3 个答案: