In Spreadsheet.com, the Primary column of a worksheet plays a special role. Entries in the primary column act as identifiers for each row, with this unique identifier appearing when rows are expanded when rows are Related between worksheets, or as the display name on Kanban cards.

Because of its unique role, the primary column is limited to data types and formula outputs that result in alphanumeric characters. For instance, primary columns cannot contain attachments files or users (either directly or as the output of formulas).

## Formulas in the primary column

Like with other columns, you can create or edit a formula for the primary column. However, the output of the formula must be alphanumeric and you may wish to pick formulas that can result in a unique identifier.

For instance, in our team's budgeting worksheet, we can set the Invoice Number column to be our primary column, with the first invoice as invoice #1, and each following invoice incrementing 1 beyond that.

This example might be even better served by the Autonumber data type, which allows you to create custom numbers with a prefix, suffix, and more

Although the primary column's data types are limited, you can use any function you'd like in the primary column.

## The column formula data type and in the primary column

If you want every value in the primary column to be determined by the same formula, a Column formula data type might be your best option. The column formula data type applies a formula for every table cell in your column and can reference other cells in each row.

For instance, instead of the simple invoice number shown above, or an autonumber style invoice number, we might want our invoice number to be a combination of multiple cell values. Like the year of the project, a prefix, an autonumber, and the first few letters of the project name. Something like "2020.NB1.Foodi". In the animation below, we change the column to a Column formula data type, then enter a formula, and apply it to the whole formula.

**Check out our article on the primary column for more information about what the primary column can do. Or our articles on formulas for more about creating and applying formulas.**