> while the primary key of the CUSTOMER table was a single CUSTOMER_ID column, the primary key of the USER table was (CUSTOMER_ID,USER_ID), and the primary key of the ADDRESS table was (CUSTOMER_ID,USER_ID,ADDRESS_ID). There were tables with 5 or 6 columns as a primary key.

Maybe they wanted to avoid maintaining a separate CUSTOMER_ADDRESS 1-to-many table or maybe it was done to make easy reverse lookups.