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# Join two datasets on a shared column in Mammoth

> Join two Views in Mammoth on one or more key columns. Pick Left, Right, Inner or Outer, choose the columns to bring in, and fix duplicate keys.
>
> Applies to: All plans. Roles: Project Editor or Project Admin. Where: View › Describe your transformation › Unify › Join

**Join** adds columns from another View (the foreign View) to the View you're in, matching rows on one or more key columns. You get one wider dataset. Open it from **Describe your transformation** (or a **+** in the Pipeline panel), then **Unify** › **Join**.

Need only one value per row, like a VLOOKUP? [Lookup](https://mammoth.io/docs/learn/clean-and-shape/lookup/) is simpler and never adds rows.

## Join two Views in four steps

1. In the **Select a view to join with** window that opens, choose the dataset and the View inside it, then click **Select "<dataset> / <View>"**. Both Views appear side by side: yours with a blue border, the foreign View with an orange one.
2. Under the key columns, pick the matching column on each side in **Key column of current view** and **Key column of foreign view**. Columns marked as an ID are listed first. To match on more columns, click **Add key columns**.
3. In **Join columns**, tick the foreign View's columns to bring in (the search box finds them). Unticked columns are left out. Each ticked column has an **as** box to rename it.
4. Choose the join type (**Left** is chosen at the start; point at a type to see an example) and click **Apply**. The Join task card lists the joined View, keys, columns, join type and prefix.

![Join panel: the current View and property_master.csv side by side, Property matched to pos_property, pms_code and property_name ticked, and Left selected](https://mammoth.io/docs/images/20261108_join-panel.webp)

## Check the result before you apply

Click **Preview** in the Join panel to see three tabs. **Views** shows both Views side by side. **Result** shows the joined rows. **Matches** counts, for each View, its rows, the rows that match and the rows with no match. It also says when rows match more than one row of the other View, which adds rows. You see a key problem before it reaches your data.

![Join preview, Matches tab: This view 480 rows, 480 that match, 0 with no match; Other view 12 rows, 12 that match, 0 with no match](https://mammoth.io/docs/images/20261108_join-matches.webp)

## Choose Left, Right, Inner or Outer

The join type decides what happens to rows whose key has no match.

| Join type | Keeps | Use it to |
| --- | --- | --- |
| **Left** | Every row in your View. Rows with no match get blank joined columns. | Add details where they exist and keep all your records |
| **Right** | Every row in the foreign View. Your rows with no match are removed. | See every reference record and which ones match |
| **Inner** | Only rows whose key is in both Views | Keep fully matched records only |
| **Outer** | Every row from both Views, with blanks where there's no match | See everything and find the gaps |

![Left Join diagram: Table 1 and Table 2 matched on Key Column, with all Table 1 rows kept in the result](https://mammoth.io/docs/images/20260511_175858_Screenshot2026-04-27110614.webp)

## Example: add customer names to orders

`Orders` has `ID`, `Customer ID` and `Total`. `Customers` has `ID` and `Name`. Join `Customer ID` to `ID`, tick `Name`, set the prefix to `Customer` and choose **Left**:

| ID | Customer ID | Total | Customer.Name |
| --- | --- | --- | --- |
| 1 | 2 | 167 | Katie |
| 2 | 1 | 98 | John |
| 3 | 4 | 98 | Raj |

Customer 3 (Priya) has no orders, so Left leaves her out. Choose **Right** to see her, with blank order columns.

## Match numbers or dates within a range

When your key is a number or a date, tick **Add range** under the key pair to match within a range, for example times within ±10 minutes. Enter how far below in **From -** and how far above in **to +**. For dates, pick the unit: milliseconds, seconds, minutes, hours, days, weeks, months or years.

![Join common keys with a number key pair and the Add range option ticked, showing From and to range fields](https://mammoth.io/docs/images/20261009_120300_Join-AddRange.webp)

## Name the joined columns with a prefix

**Prefix** ("Enter a prefix for columns") goes in front of every joined column, followed by a dot: `Name` becomes `Customer.Name`. The box starts empty. If you switch View while it's empty, Mammoth fills in that View's dataset name. Without a prefix, joined columns keep their names, and a name already in use gets a number, such as `Name 2`.

## Fix duplicate keys and extra rows

When key values repeat, the Join task card shows a warning. **More details** lists the repeated values, with **Download as CSV** and **Edit task**.

If keys repeat on both sides, each row matches several rows and the row count multiplies. Mammoth blocks a join whose result would pass 100,000 rows **and** be more than 5 times the larger View. The task then fails. To avoid it, remove duplicate keys first with **Remove Duplicate Rows**, use [Lookup](https://mammoth.io/docs/learn/clean-and-shape/lookup/), or summarise the foreign View with [Group & Pivot](https://mammoth.io/docs/learn/clean-and-shape/group-and-pivot/).

![Key column(s) have non-unique values dialog listing a duplicate entry (mita, 2), with Download as CSV and Edit task buttons](https://mammoth.io/docs/images/20260417_125955_JoininPipelineshowingnon-uniquevalues1.webp)

## Key column warnings

Mammoth warns when two keys look alike but may not match. The warnings never block **Apply**.

| Warning starts with | When |
| --- | --- |
| "order_ref is text and order_id is a number, so matching values may not join." | One key is Text, the other a Number. `000417` as text never equals `417`. |
| "The keys are stored as different types: a date never equals text or a number…" | One key is a Date and the other isn't |
| "One key is an ID and the other is not…" | Only one key is marked as an ID |
| "The keys are in different currencies…" | Both keys are Currency columns with different symbols |

Fix a type mismatch with **Convert Column Type** on either View first. See [Convert text to dates and numbers](https://mammoth.io/docs/learn/clean-and-shape/convert-column-type/).

## Troubleshooting

| Symptom | Cause | Fix |
| --- | --- | --- |
| No matches | Values differ: `US` and `United States`, spaces, case or type | Tidy both sides with **Text Formatting**, **Find & Replace** or **Convert Column Type** |
| Far more rows than expected | Repeated keys on both sides | Remove duplicates, use Lookup, or group the foreign View first |
| **Runtime error. Contact support for help.** | Result over 100,000 rows and 5 times the larger View | Remove duplicate keys first |

More help: [Fix a failing step](https://mammoth.io/docs/learn/clean-and-shape/fix-a-failing-step/).

## Related

- [Look up one value from another View](https://mammoth.io/docs/learn/clean-and-shape/lookup/): one column, never extra rows.
- [Remove duplicates and check data](https://mammoth.io/docs/learn/clean-and-shape/duplicates-and-checks/): clean keys before you join.
- [Work with text and JSON](https://mammoth.io/docs/learn/clean-and-shape/text-and-json/): make key values match.
- [Combine datasets](https://mammoth.io/docs/learn/organise/combine-datasets/): stack files from the Data Library.

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Source: https://mammoth.io/docs/learn/clean-and-shape/join.md · Updated: 2026-11-08