Check Duplicate Email Using Trigger

/*This code is check Duplicate Emil, if in the records email already exits and again you want to enter same email id then it prevent to save record and show Email Already Exits Error Message */

trigger duplicate_Email_Check on Contact (before insert, before update) {
         set<String> emailAdd = new set<String>();
         
         for(Contact con : Trigger.New){
               emailAdd.add(con.Email);    
         }
         
         List<Contact> conList = [SELECT id, Email FROM Contact where Email IN : emailAdd];
     
        if(Trigger.isUpdate){
            for(Contact con : Trigger.New){
                 if(con.Email!=null && conList.size()>0 && Trigger.oldmap.get(con.id).Email!=con.Email) {
                     con.Email.addError('Email AlReady Exits');
                  }
             }
        }
     
        if(Trigger.isInsert){
            for(Contact con : Trigger.New){
                 if(con.Email!=null && conList.size()>0) {
                     con.Email.addError('Email AlReady Exits');
                 }
            }
        }    
}

//******************************************************************************//
//If You want to Undelete Event also here is code 
trigger duplicate_Email_Check on Contact (after undelete, before insert, before update) {
    Set<String> emailAdd = new Set<String>();

    Integer contactThreshHold;

    Map<String, Integer> emailToNoOfContactsMap = new Map<String, Integer>();


    for(Contact con : Trigger.New){
        emailAdd.add(con.Email);    
    }

    for(Contact existingContact : [SELECT id, Email FROM Contact where Email IN : emailAdd]) {
        if(!emailToNoOfContactsMap.containsKey(existingContact.Email)) {
            emailToNoOfContactsMap.put(existingContact.Email, 0);
        }

        Integer emailRelatedContactCount = emailToNoOfContactsMap.get(existingContact.Email);
        emailRelatedContactCount = emailRelatedContactCount + 1;
        emailToNoOfContactsMap.put(existingContact.Email, emailRelatedContactCount);
    }


    private void checkDuplicateEmail(List<Contact> contactsList, Integer conThreshHold) {
        for(Contact con : contactsList){
            if(con.Email!=null && emailToNoOfContactsMap.get(con.Email) > conThreshHold) {
                con.Email.addError('Email already Exits');
            }
        }
    }

    if(Trigger.isInsert ) {
        checkDuplicateEmail(Trigger.New, 0);
    }

    if(Trigger.isUpdate ) {
        checkDuplicateEmail(Trigger.New, 0);
    }

    if(Trigger.isUnDelete) {
        checkDuplicateEmail(Trigger.New, 1);
    }
}

Number of Child Record Count on Parent Record Using Trigger

Number of Child Record Count on Parent Record Using Trigger

I use Account (this is parent) and Contact (this is chlid of Parent object which is Account)
this trigger work for all event like insert , update ,delete and undelete

Trigger ContactCountTrigger on Contact(After insert, After Delete, After Undelete, After Update)
{
  Set<Id> setAccountIds = new Set<Id>();

  if(Trigger.isInsert || Trigger.isUndelete){
for(Contact con : Trigger.new){
if(con.AccountID != Null)
            setAccountIds.add(con.AccountId);
    }
  }

  if(Trigger.isDelete){
     for(Contact con : Trigger.old){
    if(con.AccountID != Null){
                 setAccountIds.add(con.AccountId);
             }
     }
  }

  if(Trigger.isUpdate){
     for(Contact con : Trigger.New){
    if(con.AccountID != Null){
                 setAccountIds.add(con.AccountId);
     }
            if(Trigger.oldMap.get(con.Id).AccountId != null){
                 setAccountIds.add(Trigger.oldMap.get(con.Id).AccountId);
            }
      }
  }

 List<Account> listAccs = [SELECT id,name,number_of_contacts__c ,(Select id from contacts) FROM Account WHERE Id in : setAccountIds];

//number_of_contacts__c this is Number type Field  on account object
//contacts is a child relationship name
  for(Account acc :listAccs){
      acc.number_of_contacts__c = acc.contacts.size();
  }
  update listAccs;
}