ERROR 1052 (23000): Column 'customer_name' in field list is ambig

Jeg har 2 tabeller:

customer: som har attributterne customer_name, customer_city
haps: som kun har attributten customer_name.

I begge tabeller befinder der sig i customer_name "Bob" og "Joe".

Men naar jeg forsoeger at skrive:

select customer_name, customer_city
from customer C, haps H
where C.customer_name = H.customer_name;

saa faar jeg fejlen:

ERROR 1052 (23000): Column 'customer_name' in field list is ambiguous

Haaber nogen kan give en forklaring