Customers or Suppliers?
Do you have a table for suppliers? Do you have a table for products? Do you have a third table that associates suppliers with products? That would be the normal structure for a many-to-many relationship. So let's verify the relationship. Obviously each supplier can have many products. Can each product be associated with multiple suppliers?
Conventional options for data entry would be:
1. one form bound to the third table with comboboxes to select supplier and product
2. main form bound to Suppliers and subform bound to third table with combobox to select product
3. main form bound to Products and subform bound to third table with combobox to select supplier
Review
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