mysql - join select data from two different fields in table - mysql

Question

I have two tables:
friends table (UserID, FriendID) and
users table (UserID, FirstName, LastName).
I'm trying to do one SQL query to join and pull all records that have the UserID or FriendID inside friends table equal to the user's ID but pull the FirstName and LastName from the other UserID.
For example
friends table
UserID = 1 | FriendID = 2
UserID = 3 | FriendID = 1
user table
UserID = 1 | FirstName = "Bob" | LastName = "Hope"
UserID = 2 | FirstName = "John" | LastName = "Doe"
UserID = 3 | FirstName = "Bill" | LastName = "Murray"
If I am logged in as Bob(UserID = 1) trying to pull all of my friends user data (FirstName and LastName) in one query by checking if UserID 1 is either a FriendID or UserID inside the friends table. Then join the data for the opposite field that isn't my ID.
Any ideas?

Answer 1

Try this:
SELECT *
FROM users u
WHERE userid IN ( SELECT userid FROM friends WHERE friendid = 1
UNION ALL
SELECT friendid FROM firends WHERE userid = 1);
This will give you:
| USERID | FIRSTNAME | LASTNAME |
---------------------------------
| 2 | John | Doe |
| 3 | Bill | Murray |
SQL Fiddle Demo

Answer 2

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
On a phone, so may need syntax tweaks.
An optimiser may suggest a different join strategy based on your real data

Answer 3

If I understand your question, then this works
-- 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
Results for #logged_in = 1:
UserID FirstName LastName
2 John Doe
1 Bob Hope
Results for #logged_in = 2:
UserID FirstName LastName
2 John Doe
Test Database Create Code:
--
-- 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');