How to design an after delete trigger apex code

Question

i am new to the apex development, i am trying to learn apex triggers, how to automatically delete child records when parent record is deleted, i know through M-D relationship this can be achieved, but i need design a trigger for learning, here i have designed the trigger on Account and Related Contact_c custom object, i am getting the error, i have refered a below link but still i could not achieve the functionality

https://developer.salesforce.com/forums/?id=906F000000092e8IAA

trigger deleteChildrenOnParentDelete on Parent_Object__c (before delete) 
{   
        List<Parent_Object__c> parentList = new List<Parent_Object__c>([SELECT Id FROM Parent_Object__c]);                 
        Map<Id,Parent_Object__c> childMap = new Map<Id,Parent_Object__c>();
            For(Parent_Object__c c: parentList)
            {
                childMap.put(c.ID,c);
            } 

        List<Child_Object__c> childList = new List<Child_Object__c>();
            For(Child_Object__c c: [select Id,Parent_Object__c from Child_Object__c where Parent_Object__c IN: childMap.Keyset()])
            {
                childList.add(c);
            } 
                Delete childList;                             
}

Answer 1

To delete child records when parent is deleted in case of lookup relationship.

trigger deleteChildrenOnParentDelete on Parent_Object__c (before delete) 
{   
   if(trigger.isDelete && Trigger.isBefore){

            List<Child_Object__c> childList = new List<Child_Object__c>();
            For(Child_Object__c c: [select Id,Parent_Object__c from Child_Object__c 
                     where Parent_Object__c IN: trigger.oldMap.KeySet()])
            {
                childList.add(c);
            } 
            Delete childList;    
       }                         
}