# Join types and unions in the physical layer

*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/join-types-and-unions-in-the-physical-layer*

How to open a logical table to reach the join canvas, what inner, left, right and full outer joins keep, and how manual and wildcard unions stack same-structure tables, including Merge Mismatched Fields and the Sheet and Table Name fields a union adds.

- 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

- Open a logical table to reach the join canvas and add a join with a join clause and a join type.
- Predict which rows inner, left, right and full outer joins keep, and where nulls appear.
- Union tables with the same structure, manually or with a wildcard pattern that picks up new tables.
- Fix a union whose field names don't match, and use the Sheet and Table Name fields a union generates.
- Key terms: **join clause** (the matching fields and operator, usually =); **join type** (inner, left, right or full outer); **union** (appends rows from tables with the same columns); **wildcard union** (includes every table whose name matches a pattern with `*`); **Merge Mismatched Fields** (replaces non-matching union columns with one field).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Reach the join canvas | Double-click a logical table or select Open | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Join types | Inner, left, right, full outer | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Inner join | Unmatched values are dropped entirely | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Left join | All left values; null where the right table has no match | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Multiple join clauses | All must be true for rows to join | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Type change after joining | Breaks the join | [Join Your Data](https://help.tableau.com/current/pro/desktop/en-us/joining_tables.htm) |
| Union requirement | Same connection; ideally same fields, names and types | [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm) |
| Wildcard character | `*` in the Include search criteria | [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm) |
| Union field matching | By field name | [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm) |
| Merge mismatched fields | One field with the first non-null value per row | [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm) |
| Union-generated fields | Sheet and Table Name; usable as join keys | [Union Your Data](https://help.tableau.com/current/pro/desktop/en-us/union.htm) |

## Check yourself

- Why does a union of tables with different column names show many nulls?
- When should you choose a full outer join?
- Why can changing a join field's data type cause trouble?
- Where does the option to set up a union appear?

## Sources

- [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)
- [The Tableau Data Model](https://help.tableau.com/current/pro/desktop/en-us/datasource_datamodel.htm)
- [How Relationships Differ from Joins](https://help.tableau.com/current/pro/desktop/en-us/datasource_relationships_learnmorepage.htm)
- [Relate Your Data](https://help.tableau.com/current/pro/desktop/en-us/relate_tables.htm)
- [Data Source Page](https://help.tableau.com/current/pro/desktop/en-us/environment_datasource_page.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