# Tableau Prep unions, joins and aggregations

*Tableau Data Analyst Study Guide · Connect To and Transform Data · Last updated October 29, 2026 · https://www.certifyforce.com/study-guide/tableau-data-analyst/connect-to-and-transform-data/tableau-prep-unions-joins-and-aggregations*

Pick the Tableau Prep step a business need calls for, then configure it: Union steps and Input-step unions with mismatched fields and type conflicts, Join steps with seven join types and join-clause cleaning, Aggregate steps with grouped versus aggregated fields, and FIXED LOD calculations that keep row detail.

- Certification: Salesforce Tableau Data Analyst
- Exam section: Connect To and Transform Data
- Study guide: https://www.certifyforce.com/study-guide/tableau-data-analyst

**Official exam objective:**

> Perform data transformation in Tableau Prep: Choose which data transformation to perform based on a business scenario; Combine data by using unions; Combine data by using joins; Shape data by using aggregations; Perform filtering; Shape data by using pivots; Determine which output type to use.

## What the exam expects

- Choose the Prep transformation a scenario needs: union, join, aggregate, FIXED LOD or pivot.
- Union in a Union step or the Input step, and fix mismatched fields and type conflicts.
- Join with the right join type, read the join results and clean values that fail to match.
- Aggregate, and know whether a filter runs before or after the aggregation.
- Prep filtering, the Pivot step and output types are taught in Tableau Prep filters, pivots and output types; Desktop joins and unions in Relationships, joins, unions and data source filters.

Key terms:
- **Union:** appends the rows of one table onto another.
- **Join:** combines related tables on common fields, typically extending the table horizontally.
- **Granularity:** what one row represents, such as one row per room per day.
- **Grouped / Aggregated Fields:** the fields that make the row, and the data summarized at that level.
- **Mismatched field:** a field that did not line up across a union's tables.

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Union requirements | Same number of fields, field names and data types | [Aggregate, Join, or Union Data](https://help.tableau.com/current/prep/en-us/prep_combine.htm) |
| Union step inputs | Maximum of 10; union more in the Input step | [Aggregate, Join, or Union Data](https://help.tableau.com/current/prep/en-us/prep_combine.htm) |
| Input-step union | Same directory or sub-directory; not for Tableau extracts | [Add More Data in the Input Step](https://help.tableau.com/current/prep/en-us/prep_add_input_data.htm) |
| Wrong merged type | Separate Inputs with Different Types, change one, merge again | [Aggregate, Join, or Union Data](https://help.tableau.com/current/prep/en-us/prep_combine.htm) |
| Join defaults | Inner join; first clause from common field names | [Aggregate, Join, or Union Data](https://help.tableau.com/current/prep/en-us/prep_combine.htm) |
| Aggregate filters | Grouped field: before aggregation; aggregated field: after | [Clean and Shape Data](https://help.tableau.com/current/prep/en-us/prep_clean.htm) |
| Prep LOD support | FIXED only; no combining with another expression | [Create Level of Detail, Rank, and Tile Calculations](https://help.tableau.com/current/prep/en-us/prep_calculations.htm) |

## Check yourself

- Why can't you fix a data type in the Output step?
- When is an Input-step union unavailable?
- What do the right table's fields show in a leftOnly join?

## Sources

- [Aggregate, Join, or Union Data](https://help.tableau.com/current/prep/en-us/prep_combine.htm)
- [Clean and Shape Data](https://help.tableau.com/current/prep/en-us/prep_clean.htm)
- [Create Level of Detail, Rank, and Tile Calculations](https://help.tableau.com/current/prep/en-us/prep_calculations.htm)
- [Add More Data in the Input Step](https://help.tableau.com/current/prep/en-us/prep_add_input_data.htm)
- [Pivot Your Data](https://help.tableau.com/current/prep/en-us/prep_pivot.htm)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 11 linked practice questions, and unlimited timed mock exams are included with the Tableau Data Analyst pack (USD $19): https://www.certifyforce.com/practice-exams