AppSheet displays a drop-down for fields having a Valid_If constraint that yields a list of values.
The Valid_If expression can be:
1. A list of constant values
For example: {100, 200, 300}
Whenever a list of allowed values is provided, it is actually a syntactic shorthand for an expression that uses the IN() function. For example, {100, 200, 300} is the same as IN([_THIS], {100, 200, 300}).
2. A list of values from a column in another table
For example: LookupTable[ColumnC]
Where LookupTable is the name of the table containing the drop-down values, and ColumnC is the name of the column containing the drop-down values.
This is particularly useful if the list of allowed values can change while the app is being used.
3. A list of values from a column in another table where the rows in that table are filtered
For example: SELECT(LookupTable[ColumnC], [ReportDate] > Today() - 7)
Where LookupTable is the name of the table containing the dropdown values, and ColumnC is the name of the column containing the dropdown values. [ReportDate] > Today() - 7 is an expression which filters the rows in the table to those where ReportDate is within the last week. The resulting values in ColumnC of LookupTable become the allowed list of valid values.
4. A sorted list of values from a column in another table
For example: SORT(LookupTable[ColumnC])
Where LookupTable is the name of the table containing the dropdown values, and ColumnC is the name of the column containing the dropdown values.
The list of values is sorted in ascending order. If it should be in descending order instead, use SORT(LookupTable[ColumnC], true).
Valid_If drop-downs on Ref columns
Valid_If can also be used to modify the drop-down of a Ref column (to filter the available options, for example), but there are some additional considerations to be aware of.