SOQL.cls

trigger efficientSOQL on Invoice_Statement__c(before insert,before update)
{
  //perform SOQL query outside the for loop
  //This query runs once for all item in the Trigger.New
  for(Invoice_statement__c inv : [Select Id,Description__c,(select Id,Units_Sold__c from Line_Item__r) From Invoice_Statement__c where Id IN : Trigger.newMap.KeySet()])
  {
    for(Line_Item__c li:inv.LineItem__r)
    {
	    //do any operation
    }
  }
}