Filter values from reference table

[Rescok]

Hello to everyone. This is a very easy question but i am entering into a stupid loop. I would like to filter the possible values of a ref field when i selecting it from a input form.

In particular i want that if a value is been already used in other rows , it don't have to appare on the dropdown list on the screen. I am thinking to use the "suggested values" tab on the "auto compute" data  form. How can i do it?

Thank you to everyone

[dbaum]

Drop-down from Valid_If - AppSheet Help

Combine lists - AppSheet Help

Subtract values from a list - AppSheet Help

For example:

(expression returning master list)  // All the values
 - ThisTable[ThisColumn]            // Subtract all the values already selected
 + [_THISROW].[ThisColumn]          // Add back in the current row's saved value

[Rescok]

Hello Dbaum and thanks for the answer. However, it is not clear to me how to solve my problem and therefore I will try to better contextualize my situation.

My project has 2 tables:

- the table "desks" which contains all the tables in the restaurant with an indication of the maximum number of people it can hold;

- the table "reservations" which contains all the information regarding the number of people, the date and time of the reservation and the associated table. In particular, the table is selected from a list of tables referenced by the value "id_desk", key field of the desks table.

I would like that if on a certain date a specific table is already reserved, it will not be included in the dropdown list on the entry form.

How can i do it? It correct to insert the expression in the "suggested values" tab on the "auto compute" data  form or there is another place to use (for example "app formula")? what expression can be use in my scenario?

Thank you for your support!!!