How to link customers and suppliers in a database Some customers can be suppliers at the same time, so: how to design the database so we don't have two tables having the same columns, and, at the same time, we can link a customer and a supplier together if needed?
In the customers table I think you can add an extra column which would flag whether that customer is also a supplier.  So you could have a generic customer table (or you can name it contacts table), with a column used as a supplier flag/indicator (could be numeric: 1 or 0 or char: y or n or even boolean: true or false)