[SalesForce] How to dynamically select fields in for loop query

Something has been bugging me for a little while now, and I can't seem to find the answer anywhere…

When working with large queries, it is a good idea to use a for loop to iterate through the query results rather than just assigning the results to a list, like so:

for(Change_Order__c relatedChangeOrder : [SELECT Id,Name FROM Change_Order__c]){

    // Do stuff with relatedChangeOrder here

}

That's all fine and dandy until you want to choose specific fields to select dynamically. This is the case for me in one of my classes where I want to only select certain fields that map over properly to another object.

Obviously I can piece together a query string and use Database.query() to get the results with the fields that I want, but then I lose the for loop efficiency.

I have tried to set the Fields to a string:

String fields = 'Id,Name,Account_Number__c';

I have tried to set the Field to a List:

List<String> fields = new List<String>{ 'Id','Name','Account_Number__c' };

I've then tried inserting the fields dynamically into the query like this:

[SELECT :fields FROM Change_Order__c WHERE Active__c = TRUE]

This does not work.

Are there any other methods that people have come across? I have a query that will return a large data set and would really like to dynamically choose the fields and iterate through it with a for loop so that I don't run into SOQL limits.