How to do a lookup

I need some guidance on best way to do this in HANA

Customer    Group     

(blank)         AB          

C1234          CD         

I need to assign a Group to each Customer.  If the Customer is maintained in the table, use the Group assigned to the customer.  If the Customer is not found, assign to Group 'AB'.

Can I do this using Joins or do I have to write SQL Script code?

Thanks.