Power BI Basics›Core· 35 min read

The Data Model: Relationships Between Tables

When data lives in two tables, a relationship links them — so a sale knows which customer it belongs to.

What you will learn

Understand why data is split into tables
See a relationship on the Model view
Match a key column in both tables

Why more than one table?

Good data is usually split into separate tables to avoid repeating yourself. Instead of typing a customer name on every single sale, you keep customers in one table and sales in another, and link them. This is called a data model.

Picture two tables:

A Sales table: each row is one sale, with a CustomerID, a product and an amount.
A Customers table: each row is one customer, with a CustomerID, a name and a city.

The shared key column

Both tables share a column: CustomerID. That shared column is the key that lets Power BI connect them. A relationship says: “the CustomerID in Sales matches the CustomerID in Customers.”

Sales table	links by	Customers table
CustomerID = 7	→	CustomerID = 7 → “Asha, Delhi”
CustomerID = 7	→	(same customer)
CustomerID = 12	→	CustomerID = 12 → “Ravi, Pune”

Creating the relationship

On the far-left of Power BI Desktop there are three small icons: Report, Data, and Model. Click the Model view (it looks like little connected boxes). You will see each table as a box listing its columns. To link them:

Find CustomerID in the Sales box.
Drag it onto CustomerID in the Customers box.
A line appears between the two tables — that line is the relationship.

Power BI is smart: when it spots two columns with the same name, it often creates this line for you automatically.

A one-to-many relationship: the 1 side is Customers, the many side is Sales

Customers (1) ────────< (many) Sales

One customer  ->  can have many sales
Many sales    ->  each belong to one customer

Note: Output: The little 1 sits next to Customers and the * (many) sits next to Sales, showing one customer can appear in many sales rows. Now a chart can show “total Amount by customer City”, even though City lives in one table and Amount lives in the other — the relationship carries the link across.

Tip: This one-to-many shape is by far the most common: one customer → many sales, one product → many orders. The “one” side is the lookup list; the “many” side is the table of events.

Watch out: A relationship needs matching values in both key columns. If Sales has CustomerID 99 but Customers has no row for 99, those sales will not match a customer and may show as blank in your visuals.

Q. What lets Power BI connect a Sales table to a Customers table?

Answer: A relationship links two tables using a shared key column (e.g. CustomerID). It lets visuals combine fields from both tables correctly.

✍️ Practice

Open Model view and identify the shared key column between two of your tables.
Drag one key column onto the matching column in the other table to create a relationship.

🏠 Homework

With a Sales table and a Customers table, create a one-to-many relationship on CustomerID and describe which side is the “one” and which is the “many”.