database table design foreign key to multiple table

Question

In my PHP and psql application, I have large number of customers and service providers, both can login to the system, for login details we created a table USER but how can we map both CUSTOMER table and SERVICE_PROVIDER table in the USER table as foreign key. The structure Skeleton is

**CUSTOMER:** 
id    bigserial, 
name  character varying(100),
address character varying(350),
phone character varying(50),
etc....

**SERVICE_PROVIDER:**
id bigserial, 
name  character varying(100),
email  character varying(100),
phno  character varying(50),
first_esc  character varying(100),
sec  character varying(100),
etc


**USER**
id bigserial, 
username  character varying(100),
password  character varying(100),
customer_type  bigint 

In USER Table column customer_type may either Customer or Service Provider

How to design the table structure....

Please help me

Thanking You Anju

Answer 1

What is needed is FKs from multiple other groups (subtypes) to (supertype) user.

Every table has a statement. The rows that make the statement true go in the table. Customers are users and service providers are users. So when CUSTOMER & SERVICE_PROVIDER (etc) say something is true about a person, USER says something about them.

// "customer [id] has ..."
CUSTOMER(id,...)
    fk id references USER -- a customer is a user

// "service provider [id] has ..."
SERVICE_PROVIDER(id,....)
    fk id references USER -- a service provider is a user

// "user [id] has ..."
USER(id,...)

See this .