Relationships/Cardinality

Relationships and Cardinality  are 2 different things

Cardinality is the frequency that somethings occurs

For example

The Shop column has low  Cardinality because there is just one value (Asda)
The Transactiion ID  has high  Cardinality because each is unique
The Name has medium Cardinality because there are just 3 (Fred, Mark and James)

This is good learning video about relationships. I recommend you watch it a few times.
https://www.youtube.com/watch?v=OOs-VWf20E8

A one to many relationship (as the name suggests) must have a unique value on the parent table.
For example, the Transaction ID can support a 1:M relataionship because it is unique.
But Name cant support a 1:M relationship in the above table, because Fred has multiple rows.

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Hi cingram11 ,

I wanted to check if you had the opportunity to review the information provided. Please feel free to contact us if you have any further questions. If my response has addressed your query, please accept it as a solution and give a 'Kudos' so other members can easily find it.

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Hi cingram11 

In the context of databases and data modeling—such as in Power BI, SQL Server, or relational databases—relationships define how tables are connected to one another through common fields (typically primary and foreign keys), and cardinality describes the nature of those connections in terms of how many records in one table relate to records in another. The most common types of cardinality are:

1. One-to-one (1:1): Each row in Table A relates to exactly one row in Table B, and vice versa. This is rare and typically used when data is split into two tables for security, performance, or organizational reasons.

2. *One-to-many (1: or Many-to-one)**: A single row in the first table relates to multiple rows in the second table. This is the most common relationship. For example, a Customer table (one side) connected to an Orders table (many side), where each customer can have multiple orders.

3. Many-to-many (M:M): Rows in both tables can relate to multiple rows in the other. For example, a Students table and a Courses table, where each student can enroll in many courses and each course can have many students. This typically requires a bridge (or junction) table to model correctly in relational systems.

In tools like Power BI, setting the correct relationship and cardinality is crucial because it affects how data is filtered, aggregated, and visualized. A mismatch or incorrect relationship can lead to wrong results, ambiguous joins, or performance issues. Understanding and applying the correct cardinality helps ensure accurate and efficient data modeling.

Hi @cingram11 ,

I wanted to check if you had the opportunity to review the information provided. Please feel free to contact us if you have any further questions. If my response has addressed your query, please accept it as a solution and give a 'Kudos' so other members can easily find it.

Thank you.

Hi cingram11 ,

May I ask if you have resolved this issue? If so, please mark the helpful reply and accept it as the solution. This will be helpful for other community members who have similar problems to solve it faster.

Thank you.

Hi cingram11 

You can accept multiple solutions from helpers and it is polite to do so. Especially if you may need their help again for other questions.  Thank you.