Arbitrary "OR" SQL Queries

I was working on a Drupal migration project today using the Migrate module where I needed to import only select user roles from the source (Drupal 6) database.

The Migrate module allows for a custom query to select only the user roles that need to be imported. In my case, the two roles I wanted to import had role IDs of 4 and 6. So, how do I write a query using the Drupal Database API to do this? Turns out there's a pretty elegant answer. Rather than writing something like:

SELECT * FROM role r WHERE rid=4 OR rid=6;

The proper way of writing the select query is:

$query = parent::query();
$ored = db_or();
$ored
->condition('rid', 4)
->condition('rid', 6);
$query->condition($ored);

Note the elegant "db_or()" function that returns a DatabaseCondition object. Add the two conditions to this object, and they're automagically "or"ed.

I discovered all this via this nice blog post by Rahul Singla.