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# Column types and subtypes

> Column types in Mammoth beyond Text, Number and Date. What ID, Currency and Percentage mean, how Mammoth detects them, and how they change formats and warnings.

Every column is stored as Text, Number or Date. A Number or Text column can also carry a meaning on top of that: it is an ID, a currency amount or a percentage. Mammoth calls this the column's subtype, and treats it as part of the column's type.

A subtype never changes the stored values or what a saved task calculates. It changes the tag in the column header, which number format controls you can change, and the warnings and defaults you see when you aggregate.

## The column types

| Type | Stored as | Header tag | What it means |
| --- | --- | --- | --- |
| **Text** | Text | Aa | Free text or labels. |
| **ID** | 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** | Number | key icon | A number that identifies a row. Isn't summed or averaged by default. |
| **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.

## Read the type tag in the header

Each column header in the data grid starts with a small grey tag that shows the column's type. Hover the tag 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 a subtype

When data is loaded, 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 over a detected one. Choosing **Number** or **Text** for a column marks it as plain, and detection doesn't change it back.

## Change a column's type

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

| Where | 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 just 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 Column Type](https://mammoth.io/docs/learn/transform/column-management/#convert-column-type). |
| **Add Column** task | The new column gets the type you pick. |

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

## Number formats set by the type

A Currency, Percentage or ID column decides some of its number format. In the column menu's **Formatting** section, those buttons are greyed out and the note "Set by the column's type" appears 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 greys out 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 NUMERIC column](https://mammoth.io/docs/learn/transform/column-menu/#format-a-numeric-column).

## Aggregation warnings

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".

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Source: https://mammoth.io/docs/learn/transform/column-subtypes.md · Updated: 2026-11-03