I have a table which holds the relationship between a person and their addresses.

address_id
person_id
address_type
start_Date
end_date

A person can have more than one address_type (e.g. work, home).
If the relationship is current, then the end_date is null.

I want to create a constraint which will prevent a person from having more than one current relationship of a particular address_type.

How can I do this ?