# Special data types

[`Interval`](https://coordinatordb.com/docs/reference/data-types/special-data-types/interval) represents a signed quantity with one calendar or elapsed-time unit. Use it for date arithmetic rather than storing a duration as an unlabelled integer.

```sql
SELECT CAST('2024-01-31' AS Date) + INTERVAL 1 MONTH
FORMAT csv;
```

```text
"2024-02-29"
```

## Other special names

`Expression`, `Set` and `Nothing` have no dedicated native declarations in CoordinatorDB. Unrecognized type names follow legacy SQL affinity; declaring a column as `Nothing` does not restrict it to `NULL`.

Use [`Nullable`](https://coordinatordb.com/docs/reference/data-types/nullable) and SQL constraints for nullability, and the [supported type reference](https://coordinatordb.com/docs/reference/data-types) to choose a validated native declaration.