modifying multiple tables in one query with aprepared statement

Question

I have the following prepared query I am using with mysql and php. It is to allow users to be added to a particular category, passed in via $subcat.

$alterQuery = "UPDATE AUCTIONS SET subcat= ? WHERE article_no= ?";
   if ($altRecord = $con->prepare($alterQuery)) {
       $altRecord->bind_param("ss", $subcat, $pk);
       $altRecord->execute();
    $altRecord->close();
    echo "true";
    }

What I would like to do, in the same query, is insert a new record in the USERS table only inserting the username, after checking that the username does not already exist. Is it possible just to change $alterQuery to:

What is the best what to do this?

Check for the username, and if it == null, do a subsequent insert? can I have two different queries executed in the same if block?

edit: attaching database scheme in response to comment

AUCTIONS    

    ARTICLE_NO   varchar(20) primary key
    ARTICLE_NAME  varchar(100)
    SUBTITLE  varchar(20)
    CURRENT_BID  decimal(5,2)
    START_PRICE  decimal(5,2)
    BID_COUNT  varchar(20)
    QUANT_TOTAL  varchar(20)
    QUANT_SOLD  varchar(20)
    ACCESSSTARTS  varchar(30)
    ACCESSENDS  varchar(20)
    ACCESSORIGIN_END  varchar(20)
    USERNAME  varchar(20)  
    BEST_BIDDER_ID  varchar(20)  
    FINISHED  tinyint(4)  
    WATCH  tinyint(4)   
    BUYITNOW_PRICE  decimal(5,2)  
    PIC_URL  varchar(120)  
    PRIVATE_AUCTION  tinyint(4)
    AUCTION_TYPE  varchar(20)  
    ACCESSINSERT_DATE  varchar(20)
    ACCESSUPDATE_DATE  varchar(20)
    CAT_DESC  varchar(20)  
    CAT_PATH  varchar(20)  
    ARTICLE_DESC  text  
    COUNTRYCODE  varchar(20)
    LOCATION  varchar(20)
    CONDITIONS  varchar(20)
    REVISED  tinyint(4) 
    PAYPAL_ACCEPT  tinyint(4) 
    PRE_TERMINATED  tinyint(4) 
    SHIPPING_TO  varchar(20)
    FEE_INSERTION  decimal(5,2)
    FEE_FINAL  decimal(5,2)
    FEE_LISTING  decimal(5,2)
    PIC_XXL  tinyint(4) 
    PIC_DIASHOW  tinyint(4) 
    PIC_COUNT  varchar(20)
    ITEM_SITE_ID  varchar(20)
    STARTS  datetime  
    ENDS  datetime  
    ORIGIN_END  datetime
    SUBCAT  varchar(20)

USERS
    username   varchar(30) primary key
    firstname  varchar(30)
    lastname  varchar(30)
    flaggedauctions  varchar(30)
    lastauction  varchar(30)  
    street1  varchar(30)  
    city1  varchar(30)  
    postcode1  varchar(30)
    street2  varchar(30)
    city2  varchar(30)  
    postcode2  varchar(30)
    phone  varchar(30)  
    mobilephone  varchar(30) 
    fax  varchar(30)  
    email  varchar(30)  
    website  varchar(30) 
    bank  varchar(30)  
    banknumber  varchar(30)
    accountnumber  varchar(30)
    comments  varchar(30)

Answer 1

Why would you want to do that? Enclosing your queries in a transaction will either be a complete commit or rollback.

http://en.wikipedia.org/wiki/ACID#Atomicity

Answer 2

It will be best to create a stored procedure:

DELIMITER $$
CREATE PROCEDURE prc_add_user_to_category(_subcat VARCHAR(20), _article_no VARCHAR(20), _username VARCHAR(30))
BEGIN
        UPDATE  AUCTIONS
        SET     subcat= _subcat
        WHERE   article_no= _article_no;
        INSERT
        INTO    users (username)
        VALUES  (_username)
        ON DUPLICATE KEY
        UPDATE
        username = username;
END
$$
DELIMITER ;

and call it from your code:

$alterQuery = "CALL prc_add_user_to_category(?, ?, ?)";
   if ($altRecord = $con->prepare($alterQuery)) {
       $altRecord->bind_param("ss", $subcat, $pk, $userid);
       $altRecord->execute();
    $altRecord->close();
    echo "true";
    }

Checked and proved to compile and work against your schema on 5.2.0-falcon-alpha-community-nt-log