I am undergoing Salesforce trailhead challenges and got stuck on this one:
Create a validation rule to check that a contact is in the zip code of its account.
The challenge also has these conditions or guidelines:
A contact with a MailingPostalCode that has an account and does not match the associated Account ShippingPostalCode should return with a validation error and not be saved.
The validation rule should ONLY apply to contact records with an associated account. Contact records with no associated parent account can be added with any MailingPostalCode value. (Hint: you can use the ISBLANK function for this check)