mysql-從表中的兩個不同字段中加入選擇數據

Question

我有兩個表：

• 朋友表（ UserID ， FriendID ）和

• users表（ UserID ， FirstName ， LastName ）。

我正在嘗試執行一個SQL查詢，以聯接並提取所有在友誼表中具有UserID或FriendID記錄，這些記錄等於用戶的ID，但從另一個UserID提取FirstName和LastName 。

例如

朋友表

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

用戶表

UserID = 1 | FirstName = "Bob"  | LastName = "Hope"

UserID = 2 | FirstName = "John" | LastName = "Doe"

UserID = 3 | FirstName = "Bill" | LastName = "Murray"

如果我以Bob（ UserID = 1）的身份登錄，試圖通過檢查UserID 1是friends表中的FriendID還是UserID在一個查詢中提取所有我的朋友用戶數據（ FirstName和LastName ）。 然后將不是我的ID的相反字段的數據加入。

有任何想法嗎？

Answer 1

如果我了解您的問題，那么可以

-- 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的結果= 1：

UserID  FirstName   LastName
2       John        Doe
1       Bob         Hope

@logged_in的結果= 2：

UserID  FirstName   LastName
2       John        Doe

測試數據庫創建代碼：

--
-- 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

嘗試這個：

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

這將為您提供：

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

SQL小提琴演示

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

在電話上，因此可能需要語法調整。

優化者可能會根據您的真實數據提出不同的加入策略