What join to use?

Question

I have 2 different tables, which have just one field with same name ('username'). They're not related each other. I need with just one query to select all the rows among them both which have this field equal to a given value.

I came up with this, which is off course wrong... SELECT * FROM user AS a FULL JOIN future_user AS b WHERE a.username=x OR b.username=x

These the tables I'm talking about:

CREATE TABLE user
(
uid      mediumint(6) unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,

username varchar(15) NOT NULL,
password varchar(15) BINARY NOT NULL,
mail     varchar(50) NOT NULL,

name     varchar(50) NOT NULL,
surname  varchar(50) NOT NULL,
birth    char(10) NOT NULL,
sex      tinyint(1) unsigned NOT NULL default 1,

address  varchar(50) NOT NULL,
city     varchar(50) NOT NULL,
zip      char(5) NOT NULL,
province varchar(50) NOT NULL,
country  tinyint(3) NOT NULL,

number1  varchar(50) NOT NULL,
number2  varchar(50) NOT NULL,

last_login   TIMESTAMP,    
registered   TIMESTAMP,
online       tinyint(1) unsigned default 0,

admin           tinyint(1) unsigned default 0,
comment_allowed tinyint(1) unsigned default 0,
post_allowed    tinyint(1) unsigned default 0

) ENGINE=InnoDB;

CREATE TABLE future_user
(
username varchar(15) NOT NULL,
password varchar(15) BINARY NOT NULL,
mail     varchar(50) NOT NULL,

name     varchar(50) NOT NULL,
surname  varchar(50) NOT NULL,
birth    char(8) NOT NULL,
sex      tinyint(1) unsigned NOT NULL,

address  varchar(50) NOT NULL,
city     varchar(50) NOT NULL,
zip      char(10) NOT NULL,
province varchar(50) NOT NULL,
country  varchar(50) NOT NULL,

number1  varchar(50) NOT NULL,
number2  varchar(50) NOT NULL,

code     char(10) NOT NULL

) ENGINE=InnoDB;

Answer 1

Use a UNION :

SELECT
    fields
FROM
    user
WHERE
    user.username=x
UNION
SELECT
    fields
FROM
    future_user
WHERE
    future_user.username=x

Note that you can't do SELECT * in either because they have different fields. You'll need to return the same fields from both subqueries.

Answer 2

It sounds like what you are looking for is a Full Outer Join . Some database systems (notably MySQL) do not support such joins. For those that do, your query would be something like:

SELECT * FROM user FULL OUTER JOIN future_user
  ON user.username = future_user.username;

For those that don't, you will have to use a UNION of a LEFT JOIN and RIGHT JOIN as explained in the Wikipedia article linked above, something like (in MySQL):

SELECT * FROM user LEFT JOIN future_user
  ON user.username = future_user.username
UNION
SELECT * FROM user RIGHT JOIN future_user
  ON user.username = future_user.username;

You may want to serious consider if there is an alternative available, such as combining the tables or processing the resultsets from two separate queries on the back end. FULL OUTER JOINs are very weird to be using, and you're going to end up with a bunch of NULL field values that you'll have to check for and validate.