Symptoms

Count Drop-Down Control values on Editable List View
 

Diagnoses

One of our editable list columns has drop-down list control where we have 2 values: A and B
This column can't have null/empty value and we must have at least one row with value A selected.
So, we want to count the number of each drop-down values, to validate if the count of rows with value A selected is more than zero.
 

Resolution

1- add new column with a data label control and Call it dlRowSum
2- click on the column and go to expressions in the properties to add an expression
3- you expression should be:

SUM(If(Value = A), 1,0)

4- Add another expression and call it ListSum:

List Sum (dlRowSum)

then check in your rule the value of the ListSum expression if it's greater than zero