# Relationships versus joins in the data model

*Tableau Desktop Foundations Study Guide · Connecting to and Preparing Data · Last updated October 29, 2026 · https://www.certifyforce.com/study-guide/tableau-desktop-foundations/connecting-to-and-preparing-data/relationships-versus-joins-in-the-data-model*

The logical layer (relationships, or noodles) versus the physical layer (joins and unions), how a relationship keeps tables at their own level of detail while a join merges them up front, and when a join is still the right choice.

- Certification: Salesforce Tableau Desktop Foundations
- Exam section: Connecting to and Preparing Data
- Study guide: https://www.certifyforce.com/study-guide/tableau-desktop-foundations

**Official exam objective:**

> Create and manage the data model: Add relationships to a data source; Add joins and unions; Explain when to use a join versus a relationship.

## What the exam expects

- Add a relationship by dragging a second table onto the Data Source page canvas, and edit its matching fields.
- Open a logical table to reach the physical layer and add joins or unions there.
- Explain why a relationship keeps each table's level of detail while a join can duplicate or drop values.
- Choose a join instead of a relationship when the merged, filtered or duplicated single table is what you want.
- Relate tables whose key fields have different data types by using a calculated field.
- Key terms: **logical layer** (the Relationships canvas you see first); **physical layer** (the Join/Union canvas inside a logical table); **relationship** (a "noodle" that names matching fields but doesn't merge tables); **join** (merges physical tables into one table using a join clause and join type); **level of detail** (the row grain a table or view aggregates at).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Default canvas | Logical layer (Relationships canvas) | [The Tableau Data Model](https://help.tableau.com/current/pro/desktop/en-us/datasource_datamodel.htm) |
| Reach the physical layer | Double-click a logical table, or right-click > Open | [How Relationships Differ from Joins](https://help.tableau.com/current/pro/desktop/en-us/datasource_relationships_learnmorepage.htm) |
| Second table on the logical canvas | Forms a relationship (noodle), not a join | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Default relationship settings | Many-to-many, Some records match | [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm) |
| Relationship join type | Chosen automatically from the fields in the view | [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm) |
| Matching fields | Same data type in the underlying database; a Data Source page type change doesn't count | [The Tableau Data Model](https://help.tableau.com/current/pro/desktop/en-us/datasource_datamodel.htm) |
| Calculated fields | Relationships can be based on them | [The Tableau Data Model](https://help.tableau.com/current/pro/desktop/en-us/datasource_datamodel.htm) |
| Not allowed in relationships | Geographic fields; circular relationships | [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm) |
| Join effect | Merges tables before analysis; can duplicate, filter or add NULL rows | [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm) |

## Check yourself

- Why can a dimension-only view of related tables hide unmatched values?
- When is a join the better choice than a relationship?
- How do you reach the canvas where joins and unions are created?
- Why don't related tables need FIXED expressions to deduplicate measures?
- What relationship settings does Tableau use when no key constraints are detected?

## Sources

- [The Tableau Data Model](https://help.tableau.com/current/pro/desktop/en-us/datasource_datamodel.htm)
- [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm)
- [How Relationships Differ from Joins](https://help.tableau.com/current/pro/desktop/en-us/datasource_relationships_learnmorepage.htm)
- [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm)
- [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm)
- [Plan the Data Source](https://help.tableau.com/current/pro/desktop/en-us/datasource_plan.htm)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 5 linked practice questions, and unlimited timed mock exams are included with the Tableau Desktop Foundations pack (USD $19): https://www.certifyforce.com/practice-exams