Mark, try localizing the symbol testKey with With:

testKey = "location"; 
Select[test, (#[testKey] == "familyroom" &)] // Normal
(* {} *)

With[{testKey = "location"}, 
   Select[test, (#[testKey] == "familyroom" &)] 
 ] // Normal
(*  {<|"location" -> "familyroom", "temperaturesensor" -> "BME280"|>} *)

I don't know why the global testKey does not work. I seems to me it should.

Howdy, Mark From the examples I’ve seen in Wolfram’s post and Documentation, I think your use of “location” as a string was a problem. Try just #location... Select[test, (#location == "familyroom" &)]

Block[{test, select}, 
 test = Tabular[{<|"location" -> "study", 
     "temperaturesensor" -> "SHTC3"|>, <|"location" -> "basement", 
     "temperaturesensor" -> "BME280"|>, <|"location" -> "familyroom", 
     "temperaturesensor" -> "BME280"|>, <|"location" -> "Study", 
     "temperaturesensor" -> Missing["NotAvailable"]|>, <|"location" ->
       "DiningRoom", 
     "temperaturesensor" -> "MCP9808"|>, <|"location" -> "upstairs", 
     "temperaturesensor" -> "MCP9808"|>, <|"location" -> "Thermostat",
      "temperaturesensor" -> "DPS310"|>, <|"location" -> "FamilyTV", 
     "temperaturesensor" -> "TMP117"|>, <|"location" -> 
      "Indeterminate", 
     "temperaturesensor" -> "BME680"|>, <|"location" -> "LaniStudy", 
     "temperaturesensor" -> "BME680"|>, <|"location" -> "Basement", 
     "temperaturesensor" -> "BME680"|>}];
 select = Select[test, (#location == "familyroom" &)];
 Row[{test, Spacer[12], "\[LongRightArrow] ", select}]
 ]

Well, What I wrote works, but not if the column name was “temperature_sensor”. Then You would need to define it as a string…

Block[{test, select2}, 
 test = Tabular[{<|"location" -> "study", 
     "temperature_sensor" -> "SHTC3"|>, <|"location" -> "basement", 
     "temperature_sensor" -> "BME280"|>, <|"location" -> "familyroom",
      "temperature_sensor" -> "BME280"|>, <|"location" -> "Study", 
     "temperature_sensor" -> Missing["NotAvailable"]|>, <|"location" ->
       "DiningRoom", 
     "temperature_sensor" -> "MCP9808"|>, <|"location" -> "upstairs", 
     "temperature_sensor" -> "MCP9808"|>, <|"location" -> "Thermostat",
      "temperature_sensor" -> "DPS310"|>, <|"location" -> "FamilyTV", 
     "temperature_sensor" -> "TMP117"|>, <|"location" -> 
      "Indeterminate", 
     "temperature_sensor" -> "BME680"|>, <|"location" -> "LaniStudy", 
     "temperature_sensor" -> "BME680"|>, <|"location" -> "Basement", 
     "temperature_sensor" -> "BME680"|>}];
 select2 = Select[test, (#"temperature_sensor" === "BME280" &)];
 Row[{test, Spacer[12], "\[LongRightArrow] ", select2}]
 ]