Check if a value occues in a varialbe list

Hi Bjorn_C 

I agree with HotChilli , you need to select both postcode and city columns as matching columns, just like below. Hold on Ctrl key on keyboard and select columns in the same sequence. 

The result is like this 

Then add a custom column to get the Check result. 

if Table.IsEmpty([Tbl_Bpost]) then "NOK" else "OK"

All M code

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WsjAxMFDSUfLPLy5JzUtJVYrVQYiVpqTmJZUWpSMLRiXmpZTl5xdBlVoag0X9MlNLy1OLskEoD1nCMTGnuASvAJiKjQUA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"postal code" = _t, city = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"postal code", type text}, {"city", type text}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"postal code", "city"}, Tbl_Bpost, {"Postcode", "Plaatsnaam"}, "Tbl_Bpost", JoinKind.LeftOuter),
    #"Added Custom" = Table.AddColumn(#"Merged Queries", "Check", each if Table.IsEmpty([Tbl_Bpost]) then "NOK" else "OK")
in
    #"Added Custom"

Best Regards,
Community Support Team _ Jing
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