best way to check table for variable

[android6011]

I want to make sure a username doesnt exist before inserting it into a table, what is the best way for this? should i pull the value from the table as a variable and loop check it or pull it as an array? I'm just looking for the most efficient and best method. thanks

[awpti]

<?php

$res = mysql_query("SELECT * FROM my_user_table WHERE Username = '{$some_username}');

if(mysql_num_rows($res) === 0)
{
    //create new user
}
else
{
   //User exists.
}

 

 

You get the idea