Need help with a T-SQL query

I have a table of locations (addresses) similar to this:

location_id int identity (primary key)
contact_id int (foreign key to contact table; one-to-many relationship from contacts to locations)
<address info>
is_primary int (1 if this is the contact's primary address, zero otherwise)

There can be only one primary address per contact. Due to an error with a recent data upload, this开发者_JAVA百科 rule has been violated and I now have several contacts with more than one primary address. I need help formulating a query to find out which ones. Ideally I'd like the output to be:

contact_id       count(is_primary)

I have no idea how to do it and would appreciate your assistance. Thank you.

SELECT contact_id, SUM(IS_PRIMARY) 
FROM table_name 
GROUP BY contact_id 
HAVING SUM(IS_PRIMARY) > 1