problem in mysql queries in using union - php

Question

i was looking for a way to combine different mysql queries in a php file so this is my code :
$sql_query = "SELECT b.*,
u.username AS MY_Sender
FROM table_users u,
table_blogs b
WHERE b.reciever = '0'
AND
u.user_id = b.sender
UNION
SELECT b.*,
u2.username AS MY_Recipient
FROM table_users u2,
table_blogs b
WHERE b.reciever != '0'
AND
u2.user_id = b.reciever
";
this code works fine unless it cant fetch MY_Recipient
in the above code i need to fetch both sender of blog post and the receiver
is it wrong to use Union to do so ?!

Answer 1

I have made a guess at your table structure, and produced something similar. Right or wrong, it might at least help arrive at a suitable solution for you.
Two tables, users and blogs:
CREATE TABLE `users` (
`id` int(11) NOT NULL auto_increment,
`username` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
CREATE TABLE `blogs` (
`id` int(11) NOT NULL auto_increment,
`sender` int(11) NOT NULL,
`receiver` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
Add some users:
INSERT INTO `users` (username) VALUES
('Alice'), ('Bob'), ('Carol'), ('Eve');
Add blog entries for some users:
INSERT INTO `blogs` (sender, receiver) VALUES
(1,2), (2,1), (3,4), (4,3), (1,4), (4,1);
For each blog entry, list the sender and receiver:
SELECT
b.id,
b.sender AS sender_id,
b.receiver AS receiver_id,
us.username AS sender_name,
ur.username AS receiver_name
FROM blogs AS b
JOIN users AS us ON us.id = b.sender
JOIN users AS ur ON ur.id = b.receiver
ORDER BY b.id;
+----+-----------+-------------+-------------+---------------+
| id | sender_id | receiver_id | sender_name | receiver_name |
+----+-----------+-------------+-------------+---------------+
| 1 | 1 | 2 | Alice | Bob |
| 2 | 2 | 1 | Bob | Alice |
| 3 | 3 | 4 | Carol | Eve |
| 4 | 4 | 3 | Eve | Carol |
| 5 | 1 | 4 | Alice | Eve |
| 6 | 4 | 1 | Eve | Alice |
+----+-----------+-------------+-------------+---------------+
UPDATE 1
table_blogs should probably look like this:
CREATE TABLE IF NOT EXISTS `table_blogs` (
`bid` int(10) NOT NULL AUTO_INCREMENT,
`content` varchar(255) DEFAULT NULL,
`date` varchar(14) DEFAULT NULL,
`sender` int(10) NOT NULL,
`reciever` int(10) NOT NULL,
CONSTRAINT `fk_sender`
FOREIGN KEY (`sender` )
REFERENCES `table_users` (`id` )
ON DELETE CASCADE
ON UPDATE CASCADE,
CONSTRAINT `fk_receiver`
FOREIGN KEY (`receiver` )
REFERENCES `table_users` (`id` )
ON DELETE CASCADE
ON UPDATE CASCADE,
PRIMARY KEY (`bid`)
) ENGINE=MyISAM AUTO_INCREMENT=1 ;
The CONSTRAINT clauses will prevent inserting values for users which don't exist, and will delete entries when users are deleted from the user table.
UPDATE 2
I think this is what you want, but as KM and bobince have stated in the comments, it violates foreign key constraints, which is not really a good idea. So, assuming no foreign key constraints, here's some additional inserts and a modified query:
INSERT INTO `blogs` (sender, receiver) VALUES
(1,0), (0,1), (4,0), (0,4), (2,0), (0,2);
SELECT
b.id,
b.sender AS sender_id,
b.receiver AS receiver_id,
IFNULL(us.username, ur.username) AS sender_name,
IFNULL(ur.username, us.username) AS receiver_name
FROM blogs AS b
LEFT JOIN users AS us ON us.id = b.sender
LEFT JOIN users AS ur ON ur.id = b.receiver
ORDER BY b.id;
+----+-----------+-------------+-------------+---------------+
| id | sender_id | receiver_id | sender_name | receiver_name |
+----+-----------+-------------+-------------+---------------+
| 1 | 1 | 2 | Alice | Bob |
| 2 | 2 | 1 | Bob | Alice |
| 3 | 3 | 4 | Carol | Eve |
| 4 | 4 | 3 | Eve | Carol |
| 5 | 1 | 4 | Alice | Eve |
| 6 | 4 | 1 | Eve | Alice |
| 7 | 1 | 0 | Alice | Alice |
| 8 | 0 | 1 | Alice | Alice |
| 9 | 4 | 0 | Eve | Eve |
| 10 | 0 | 4 | Eve | Eve |
| 11 | 2 | 0 | Bob | Bob |
| 12 | 0 | 2 | Bob | Bob |
+----+-----------+-------------+-------------+---------------+

Answer 2

The field name should be the same
Rename My_sender and My_Recipient to "User" and the union will work.

Answer 3

What are you trying to do? You say there are two queries there, but it looks like the same query to me, just one of them having a different table alias.
The only purpose I can see for the UNION is to put all the rows with a zero-receiver before those without. But you can do that more simply by using a computed ORDER BY:
SELECT b.*, u.username
FROM table_blogs AS b
JOIN table_users AS u ON u.user_id=b.sender
ORDER BY b.receiver<>0
if there are no negative receiver IDs, you could change that to ORDER BY b.receiver as 0 would always come first, which would then be possible to index if you needed to;
ANSI JOIN is generally considered more readable than the old-school method of implicit joins in the WHERE conditions;
<> is preferable to !=, which is a non-standard MySQL synonym;
check the spelling of receiver.

Answer 4

For a union to work, the two select statements should return identical columns. This is where the query is failing.

Answer 5

You can do this in a single query, but if you want to use unions, the problem is that both queries need to have the same column names:
select b.*, u.username AS username, "sender" as type ...
select b.*, u2.username AS username, "recipient" as type...