When querying for data, there are situations where the returned data-set is empty for certain time ranges. Such a situation occurred in **Vega** (returned an empty array) and I tried to create a condition using **expressions** to check if the data-set is empty but it doesn't seem to work.

`length(data('table')) == 0 ? '-' : datum.sum`

Adding the above test condition in a **formula transform** says `Undefined data set name: "table"`

. Nothing seems to change when it is used inside a **mark** *as a signal for a field*.

How to check if the returned data-set is empty so that a condition can be applied to do a specific task accordingly? Your kind help would be appreciated.

**Note:** Vega v 4.3.0