Compare two list

Hi, I have two table in my appsheet, the first table has the column of Room is ordered, it value is ref type which references to the second table which has the column of Room List (Enum type). I want to compare two table and show me which room isn’t being ordered.

Please take a look at

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I tried this function in Valid if: ({SELECT(Table 1[Room list])} - {SELECT(Table 2[Room is ordered ])})
And it showed “ LIST has elements of mismatched types appsheet”

Room is orderd column value (Ref type) is ref of Room list column value (Text type)

I don’t know what is the problem. Please help me out. Thanks!

There is nothing correct with that expression. I STRONGLY encourage you to spend more time reading the available documentation on expressions.