mysql - 连接表中两个不同字段的选择数据

Question

我有两张桌子：

• 朋友表格（UserID，FriendID）和

• 用户表（UserID，FirstName，LastName）。

我正在尝试执行一个SQL查询来加入并提取所有记录，其中的朋友表中的UserID或FriendID等于用户的ID，但请提取FirstName和{来自其他LastName的{​​1}}。

例如

朋友表

UserID

用户表

UserID = 1 | FriendID = 2
UserID = 3 | FriendID = 1

如果我以Bob（UserID = 1 | FirstName = "Bob" | LastName = "Hope" UserID = 2 | FirstName = "John" | LastName = "Doe" UserID = 3 | FirstName = "Bill" | LastName = "Murray" = 1）的身份登录，尝试通过检查{{1}来在一个查询中提取所有朋友的用户数据（UserID和FirstName）在Friends表中，1是LastName或UserID。然后加入不是我的ID的相反字段的数据。

有什么想法吗？

Answer 1

如果我理解你的问题，那么这可行

-- set the id of the logged in user
set @logged_in = 1;

-- select all the fields from the user table
select users.* from users
-- joined the friends table on the `FriendID`
inner join friends on friends.FriendID = users.UserID
-- filtered by `UserID` on friends table matching logged in user
and friends.UserID = @logged_in -- logged in id
-- union-ed with the users table
union select * from users
-- filtered by the `UserID` being the logged in user
where users.UserID = @logged_in -- logged in id

@logged_in = 1的结果：

UserID  FirstName   LastName
2       John        Doe
1       Bob         Hope

@logged_in = 2的结果：

UserID  FirstName   LastName
2       John        Doe

测试数据库创建代码：

--
-- Table structure for table `friends`
--

CREATE TABLE IF NOT EXISTS `friends` (
  `UserID` int(11) NOT NULL,
  `FriendID` int(11) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

--
-- Dumping data for table `friends`
--

INSERT INTO `friends` (`UserID`, `FriendID`) VALUES
(1, 2),
(3, 1);

-- --------------------------------------------------------

--
-- Table structure for table `users`
--

CREATE TABLE IF NOT EXISTS `users` (
  `UserID` int(11) NOT NULL,
  `FirstName` varchar(50) NOT NULL,
  `LastName` varchar(50) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=latin1;

--
-- Dumping data for table `users`
--

INSERT INTO `users` (`UserID`, `FirstName`, `LastName`) VALUES
(1, 'Bob', 'Hope'),
(2, 'John', 'Doe'),
(3, 'Bill', 'Murray');

Answer 2

试试这个：

SELECT *
FROM users u
WHERE userid IN ( SELECT userid FROM friends WHERE friendid = 1
                  UNION ALL
                  SELECT friendid FROM firends WHERE userid = 1);

这会给你：

| USERID | FIRSTNAME | LASTNAME |
---------------------------------
|      2 |      John |      Doe |
|      3 |      Bill |   Murray |

SQL Fiddle Demo

Answer 3

Select b.uid as userid, a.firstname, a.lastname 
from user a
Inner join (select friendid as uid from friends where userid=:currentUser
Union select userid as uid from friends where friendid=:currentUser) b

在手机上，可能需要进行语法调整。

优化工具可能会根据您的实际数据建议不同的加入策略