Currently there is not a way to filter values in dropdowns this way but this would be great for a future improvement!
You can use references to do something similar. I have created an example for you at the link below:
First, you select a Country in the Countries table. The Cities table will populate with cities linked to that Country. Then, you can select a city. You’ll notice that the Workshops table updates as you make selections. After selecting your Country and City, enter a value in the Topic column of the Workshops table.
After adding a value, hit enter to create the record. The Country and City columns will auto populate based on the country and city selected in the top two tables.
To set this up, first create a table titled ‘Countries’. This table will only have a single column and the Countries will be listed.
Next, create a table titled ‘Cities’. This table will have two columns. The first column will be ‘Country’. This is a reference column that pulls data from the Countries table. Our second column is ‘City’. Here, we link each city the Country it is in.
Next, open the table configuration panel on the right hand side of the page and under the Widget menu, select ‘Change Widget’. We can now choose to Select By Countries. This means that when we select a country in the Countries table, our Cities table will update to only show cities in that country.
You can add cities to the Cities table now and it will auto-assign the country based on what country is selected in the Countries table.
We can hide the Country column from our Cities table since we can see which country is selected in the Countries table. To hide, hover your mouse over the column header then click the arrow icon that appears.
Then select ‘Hide Column’ from the dropdown.
Last, we have our Workshops table. This table contains a Country column and City column - both are reference columns. The Country column points to the Countries table and the City column points to the Cities table. We are going to link this table via the City column so under ‘Select By’, I selected Cities.
Now, this table will update based on what city is selected in the Cities table. The City column in the Workshops table will also auto populate based on the city that is selected in the Cities table.
As you see in the screenshot above, the Country column does not populate. We can do this with a formula.
Here, we are using dot notation and using the reference column City to pull information from the Cities table. Specifically, we want to pull the value from the Country column of the Cities table.
Now, both City and Country auto populate.
Please let me know if you have any other questions.