Help needed: 1 login user has Multi Customers

Hello to all,
I need some help with solving my thinking problem.
1 user logons tot the website, but has multiple customers.
In the Table: Debtor_No are stored (comma seperated) all debtor_number the loggedin user may see: Example: 01,2780,2782,2798,2820
Debtor_numbers will be the key to other tables,
What is for me the best way to setup this model?

Is it by query's in SQL or can i start with Session variables?
Does somebody know an article or sollution,
Kind regards,
Ivan