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# Set column mapping and type rules for new data in Mammoth

> Tell Mammoth how to fit new files into a dataset: what happens when column types don't match, and aliases that match columns with different names.
>
> Applies to: Roles: Edit-level access to the dataset. Where: Data Library › dataset panel › Add data › Column mapping

Column mapping rules tell Mammoth how to fit every new file into a dataset: what to do when a column's type doesn't match, and which other names a column can arrive under. Open the dataset's **Add data** dialog in the **Data Library** and select the **Column mapping** row below the file drop zone. You need Edit-level access to the dataset (see [Add rows to a dataset or replace its data](https://mammoth.io/docs/learn/get-data-in/add-or-replace-data/)).

## Open the Combine / Replace Settings dialog

The **Column mapping** line shows whether the dataset's incoming-data rules are **Default** or **Custom**:

- **Default:** Mammoth maps columns by matching names and converts mismatched column types to text.
- **Custom:** the dataset has saved, non-default mapping rules.

![Column mapping row showing Default, with a pencil icon at the right to edit](https://mammoth.io/docs/images/20260505_060023_batch_col_mapping_area1.webp)

Select the row (a pencil icon appears when you hover) to open the **Combine / Replace Settings** dialog.

![Combine / Replace Settings dialog with If column type doesn't match set to Convert destination column to text and Column Mapping set to Map with existing columns](https://mammoth.io/docs/images/20260505_060336_combine_replace_setting_modal1.webp)

## If column type doesn't match

| Option | What it does |
| --- | --- |
| **Convert destination column to text** (default) | Changes the destination column's type to Text so all incoming values fit. Broadest compatibility, with no data lost. |
| **Convert incompatible values to null** | Keeps the destination column's type. Incoming values that can't be cast to it are stored as `null`. |
| **Do not update the dataset** | Stops the operation if any column type mismatch is found. Use it when strict type integrity matters. |

## Match columns with different names using aliases

| Option | What it does |
| --- | --- |
| **Map with existing columns** (default) | Matches incoming columns to destination columns by exact display name. No manual mapping needed. |
| **Create new column mapping** | You define one or more name aliases per destination column. Mammoth uses them to find the right incoming column even when the name differs between files. |

Use aliases when the same column has different headers in different files, for example `Cust_ID`, `CustomerID` or `Customer id`. With **Create new column mapping** selected, a table lists every destination column with its aliases:

| Element | What it does |
| --- | --- |
| Alias pills (for example, `Customer id ×`) | The names Mammoth looks for in incoming data. Click a pill to edit its name; click **×** to remove it. Several aliases are joined with "or", so any match works. |
| **+** button | Adds a new alias for this column |
| Toggle | Turns mapping on or off for this destination column |
| Type icon and column name (right side) | The destination column's data type and name |

Tick **All destination columns must be mapped** to add new data only when its number of columns matches the existing dataset.

Click **Save changes**. The **Column mapping** line now shows **Custom**, and future loads use your saved rules.

## Related

- [Add rows to a dataset or replace its data](https://mammoth.io/docs/learn/get-data-in/add-or-replace-data/): the **Add data** dialog.
- [Fix a pending batch](https://mammoth.io/docs/learn/get-data-in/pending-batches/): map the columns of one file that didn't fit.
- [Add another dataset's rows to a dataset](https://mammoth.io/docs/learn/get-data-in/add-from-another-dataset/): map columns between two datasets.

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Source: https://mammoth.io/docs/learn/get-data-in/column-mapping-rules.md · Updated: 2026-11-08