When you add Interval merge as an action to a table, the “Values in column” option only shows columns from the current table, while the “are greater or equal to” and “and less than” options show only columns from the “Table to merge”. I have a situation where I wanted the table to which I added the interval merge action to appear in the “are greater or equal to” and “and less than” options, and the table to merge in the “Values in column” option. The main table contained information on employees, including “Valid from” and “Valid to” columns, and I wanted to add info from a second table where the employeeids were equal and the date in a column from the second table was in between the valid from and valid to dates. I couldn’t easily interval merge from the second table, because not all employees appeared in it.
I understand this way of interval merging is probably uncommon, but I imagine it wouldn’t be difficult or harmful to give the user the option to choose which table appears under which options.