Display Alert box through the trigger - salesforce
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Can somebody tell me how can we display an alert message (not error message) through the trigger. I have written a trigger that checks for duplicate accounts in the system. At present my trigger gives the error message to the user telling that there is a duplicate account. But if the user changes the value of a field "Is record near to duplicate" to YES, the trigger allows the user to save the record.

But, I want to display the error message in an alert pop up box like "Account with this Name exists,are you sure you want to continue" and then user clicks Yes and the record gets created. Any thoughts on how I can do this. My code is below:

    trigger TRG_AccountPhysicaladdressDuplicatePreventer on Account(before insert, before update)  
    {  
    Map<String, Account> accountMap = new Map<String, Account>();  
    Map<String, Account> accountMap2 = new Map<String, Account>();    
    Map<String, Account> accountMap3 = new Map<String, Account>();    
    List<Account> listAccount= new List<Account>();    
 
    for(Account acc: [Select Physical_Street__c,Phone,Name, Id from Account])  
    {  
     accountMap.put(acc.Physical_Street__c,acc);  
     accountMap2.put(acc.Phone,acc);  
     accountMap3.put(acc.Name,acc);  
    }  
 
    for(Account account: System.Trigger.New)  
    {  
     if(accountMap.containsKey(account.Physical_Street__c)==accountMap2.containsKey(account.Phone))
     if(accountMap.containsKey(account.Physical_Street__c)==accountMap3.containsKey(account.Name))  
     if(account.Is_record_near_to_duplicate__c.equals('No'))  
      {  
       account.addError('Agency with this Name, Physical Street and Phone Number already      exists. If you still wish to create the agency change the value of field "Is Record Near To Duplicate" to YES');  
      }  
    }