Dependent dropdowns allow you to limit the values displayed in one dropdown based on the value selected in another dropdown. For example, this Lead Tracker sample asks for the 'Lead Region' (America, Asia, Europe) and then for a 'Country' within that region.
This actually requires relatively complex logic, but AppSheet tries to make it simple. Dependent dropdowns are driven by a separate lookup table.
In the sample, there is a separate 'Regions' lookup table with two columns: 'Region' and 'Country'. This acts as the lookup table for allowed combinations of regions and countries. Here is the table data used in the sample.
The 'Lead Region' column has a regular Valid_If constraint: Regions[Region]. Therefore, when a new entry is being added, the input for this column shows three choices: America, Asia, and Europe.
Likewise, the 'Country' column also specifies a similar Valid_If constraint: Regions[Country]. However, because it follows the 'Lead Region' column and because both specify columns from the same lookup table 'Regions', AppSheet recognizes the intent and implements a dependent dropdown.
Internally, AppSheet creates an expression to capture the allowed set of values for the 'Country' column. The expression must say (in English!):
Look at the Regions table
Filter the rows to make sure that the Region column of the table matches the value in the 'Lead Region' column of the row being edited in the form
Now extract the 'Country' column from those filtered rows
Eliminate any duplicates --- these are the allowed countries!
Recompute this list each time the 'Lead Region' is changed
Strictly for an expression afficionado, here is the full underlying AppSheet expression: IN( [_THIS], SELECT(Regions[Country], [_THISROW].[Lead Region] = [Region]))
While most app creators will never need to express something this complicated, you could in fact provide this expression as a Valid_If constraint. It is useful to know for advanced use cases. For example, instead of using an equality condition, an app creator could use inequality or richer expressions to build very expressive dynamic dropdowns.
Disabling Dependent Dropdowns
When the AppSheet platform finds several columns that have Valid_If constraints from the same lookup table, it assumes that the desired behavior is to have depedent dropdowns. Occasionally, this may not be what you wanted.
In these cases, provide an explicit SELECT() expression instead of just providing a column set expression. In other words, instead of Regions[Region], use SELECT(Regions[Region], true).