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# Column types in Mammoth: IDs, currency and percentages

> What Mammoth's column types mean: Text, Number, Date, ID, Currency and Percentage. How Mammoth detects them, how to change them, and the warnings they drive.

Every column is stored as Text, Number or Date. A Number or Text column can also carry a meaning on top of that, called its subtype: it is an ID, a currency amount or a percentage. You change a column's type from **Column Manager**, a **Convert Column Type** task or an **Add Column** task. The subtype never changes stored values; it changes the header tag, which number formats you can change, and the warnings you see.

## Each column type and what it means

| Type | Stored as | Header tag | What it means |
| --- | --- | --- | --- |
| **Text** | Text | Aa | Free text or labels. |
| **ID** (under Text) | Text | key icon | A value that identifies a row, like a customer or order ID. Kept as text, so leading zeros stay. |
| **Number** | Number | 123 | Numeric values you can calculate with. |
| **ID** (under Number) | Number | key icon | A number that identifies a row. Isn't summed or averaged by default; adding or averaging it has no meaning. |
| **Currency** | Number | $ icon | An amount of money. |
| **Percentage** | Number | % icon | A share or rate. |
| **Date** | Date | calendar icon | A calendar date or timestamp. |

Date columns have no subtypes. A subtype doesn't change what a saved task calculates.

## Read the type tag in a column header

Each header in the data grid starts with a small grey tag showing the column's type. Hover it for its meaning, for example "ID (Number). A number that identifies a row, like a customer or order ID. Isn't summed or averaged."

The tag is a label only. To change the type, use one of the places below.

## How Mammoth detects IDs, currency and percentages

When data loads, Mammoth looks for IDs, currency amounts and percentages. It sets a subtype only when it is confident, and only one that fits the column's storage type. In the type menu, the entry Mammoth chose is marked **detected**.

A type you choose always wins. Choosing **Number** or **Text** marks a column as plain, and detection doesn't change it back.

## Change a column's type or meaning

The type menu is the same everywhere. Its first level is **Number**, **Text** and **Date**:

- **Number** opens **Number**, **ID**, **Percentage** and **Currency**.
- **Text** opens **Text** and **ID**.
- **Date** is a single option.

While a change is pending, the menu head shows **Reset** to put back the type the column had.

| Where you change it | What the change affects |
| --- | --- |
| **Column Manager** in the data editor (click the column count in the bottom bar, then **Column Manager**) | Choose **This view only** to change only this View, or apply it to the whole dataset ("Modifies the dataset schema for all views."). |
| Column Manager in the dataset's settings | The dataset, so every View sees the new type. |
| **Convert Column Type** task | This View, from that task onward. Picking another entry under the column's own type (for example **Percentage** for a Number column) changes only the meaning, not the values. See [Convert text to dates and numbers](https://mammoth.io/docs/learn/clean-and-shape/convert-column-type/). |
| **Add Column** task | The new column gets the type you pick. See [Add, split, copy and delete columns](https://mammoth.io/docs/learn/clean-and-shape/manage-columns/). |

If a subtype can't be saved, you see "Couldn't change what this column means".

## Why some number format buttons are locked

A Currency, Percentage or ID column decides part of its number format. In the column menu's **Formatting** section, those buttons are greyed out with the note "Set by the column's type" below them.

| Type | Controls you can't change in the column menu |
| --- | --- |
| **Currency** | Percentage. If the type was set for the whole dataset, the currency symbol too. |
| **Percentage** | Currency. If the type was set for the whole dataset, the percentage button too. |
| **ID** (Number) | Thousands separator, so codes show as typed. |
| **Number**, set as plain for the whole dataset | Currency and percentage. |

A type that Mammoth only detected locks less. A detected Currency greys out only the percentage button, a detected Percentage only the currency picker, and a detected ID nothing. Decimal places are always yours to change. See [Format a number column as currency, percent or decimals](https://mammoth.io/docs/learn/clean-and-shape/rename-format-sort/).

## Warnings when you sum or average an ID or a percentage

Summing or averaging an ID adds up codes, and summing percentages rarely means anything. Mammoth warns you where you pick an aggregation:

| Where | ID column with Sum or Average | Percentage column with Sum |
| --- | --- | --- |
| Explore card **Show values by** | "Adding or averaging it adds up the codes. Did you mean Count distinct?" | "Adding percentages rarely means anything. Did you mean Average?" |
| **Group & Pivot** task | Same text, with **Swap to Count distinct** and **Keep** | Same text, with **Swap to Average** and **Keep** |
| **Create Metric** from a column header | "Sum adds up the codes." or "Average of the codes is not a quantity." | "Adding percentages rarely means anything. Average usually does." |

Each warning starts with the column name, for example "order_id is an ID." The warnings don't block you. In Group & Pivot, **Keep** dismisses the warning for that aggregation and keeps your choice.

When you pick a column for Sum or Average in an Explore card, ID columns are listed last with the hint "it's an ID".

## Every warning a column's meaning can trigger

Some functions use a column's meaning to question a choice that is probably a mistake. Most warnings start with a bold lead such as "**order_id is an ID.**" None of them blocks **Apply**.

| Function | Warning | When |
| --- | --- | --- |
| Math Function | "IDs are labels, not amounts, so doing arithmetic on them isn't meaningful." | A formula (in Formula mode) uses an ID column. |
| Math Function | "This formula mixes an amount of money with a percentage. Check that the result is what you mean." | A formula (in Formula mode) uses a Currency and a Percentage column together. |
| Window Function | "IDs are labels, not amounts, so a running total or ranking of them isn't meaningful." | A sum, average or ranking runs over an ID column. |
| Group & Pivot | "Adding or averaging it adds up the codes. Did you mean Count distinct?" | Sum or Average of an ID column. Offers **Swap to Count distinct** or **Keep**. |
| Group & Pivot | "Adding percentages rarely means anything. Did you mean Average?" | Sum of a Percentage column. Offers **Swap to Average** or **Keep**. |
| Join, Lookup | Key type and meaning warnings | The keys are stored as different types, one is an ID and the other isn't, or they are in different currencies. See [Join two datasets on a shared column](https://mammoth.io/docs/learn/clean-and-shape/join/). |
| Unpivot | "The new value column does not carry over the meaning of the columns you stack (an ID, a currency or a percentage). It starts as a plain column." | A stacked column has a meaning. |

[Data Check](https://mammoth.io/docs/learn/clean-and-shape/duplicates-and-checks/) also suggests checks for ID, Percentage and Currency columns.

## Troubleshooting column types

| Symptom | Cause | Fix |
| --- | --- | --- |
| "Couldn't change what this column means" | The subtype couldn't be saved | Try again from Column Manager or a Convert Column Type task |
| A number format button is greyed out with "Set by the column's type" | The column's type decides that setting | Change the column's type |
| Leading zeros disappeared from a code | The code was converted to a Number | Use **ID** under **Text** to keep leading zeros |
| Detection keeps a column plain | You chose **Number** or **Text**, which wins over detection | Pick the subtype you want from the type menu |

## Related

- [Convert text to dates and numbers](https://mammoth.io/docs/learn/clean-and-shape/convert-column-type/): the Convert Column Type task and its format codes.
- [Rename, format and sort columns](https://mammoth.io/docs/learn/clean-and-shape/rename-format-sort/): number and date display formats.
- [Group, pivot and unpivot](https://mammoth.io/docs/learn/clean-and-shape/group-and-pivot/): where the Sum and Average warnings appear.
- [Calculate with formulas](https://mammoth.io/docs/learn/clean-and-shape/formulas/): Math Function and Window Function warnings.
- [Metrics](https://mammoth.io/docs/learn/explore/metrics/): Create Metric from a column header.

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