Challenge 1: Relating Products and Sales — Solution Walkthrough

Steps:
  1. Ensure both Products and Sales have already been added to the
     Data Model (via "Add this data to the Data Model" when creating
     a PivotTable, or through Power Query's Close & Load To dialog).
  2. Power Pivot tab > Manage, to open the Power Pivot window.
  3. Switch to Diagram View.
  4. Drag from Sales[ProductID] onto Products[ProductID] (or the
     reverse — the direction dragged doesn't change the relationship
     type, only which table Excel treats as the starting point in the
     dialog).
  5. Confirm the relationship in the dialog that appears, then close
     Power Pivot.

Relationship type: one-to-many (specifically, Products is the "one"
side, Sales is the "many" side).

Why:

Each ProductID appears exactly ONCE in Products (assuming no
duplicates — see Exercise 3 for what happens if that assumption is
violated), but the SAME ProductID can appear many times in Sales, once
per individual sale of that product. One product genuinely
corresponds to many sales rows — that's the definition of a
one-to-many relationship, the same underlying logic as a foreign key
in a real database, where Sales[ProductID] "points at" a single
matching row in Products.

This is NOT a many-to-many relationship, because a many-to-many
relationship would require BOTH sides to potentially repeat the key
— which isn't the case here, since Products[ProductID] is meant to be
unique.

Notes:
- Once related this way, dragging Category (from Products) and
  Amount (from Sales) into the same PivotTable works immediately,
  with Excel resolving the join automatically behind the scenes —
  no merged table or VLOOKUP formula needed anywhere.