Batch Apex declaration

global class CL_BATCH_update_Purchases implements Database.Batchable<sObject> {
	
	public CL_BATCH_update_Purchases(){} 
	
	global Database.QueryLocator start(Database.BatchableContext BC){
		String query='Select Id from SFDC_Purchase_Order__c';
		return Database.getQueryLocator(query);
	}

	global void execute(Database.BatchableContext BC,List<SFDC_Purchase_Order__c> scope){		
		List<SFDC_Purchase_Order__c> listOrden=new List<SFDC_Purchase_Order__c>();
				
		for(SFDC_Purchase_Order__c orden : scope){
			listOrden.add(orden);		
      	}      
      	update listOrden;
	}

	global void finish(Database.BatchableContext BC){}
}