How to compare old field value with the New field value in Salesforce?

How to compare old field value with the New field value in Salesforce?

Here you can compare the values with  trigger.oldMap . Here is the sample example on contact email field.

trigger Email_Check_On_Contact on Contact (before update) {
  Map<Id,Contact> o = new Map<Id,Contact>();
    o = trigger.oldMap;
    for(Contact newcont: trigger.new)
    {
      
        if(newcont.Email != o.get(newcont.Id).Email)
        {
            newcont.Email.addError('Email cannot be changed');
        }
    }
}

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