Insert statement with a conditional target column - sql

Question

I am trying to store sensor data recorded to the appropriate target column, depending on what the latest settings are in the settings table (represented as #Unit in the code). The code I have so far gives me a syntax error at line 19 near 'Time', which I am not sure as to why. Any ideas?
CREATE PROCEDURE StoreTemp
#Temperature float,
#Seconds int,
#SessionId int,
#DatasetId int
AS
DECLARE
#Unit varchar(20),
#TempCol varchar(20)
SELECT #Unit = RecordingUnit from ReadLastUnit
SELECT #TempCol = (case #Unit when 'Celsius' then 'Temperature_C' else 'Temperature_F' END)
INSERT INTO DATASET (#TempCol, Time)
VALUES (#Temperature, GETDATE())
GO

Answer 1

You can't use a variable to specify a column name in an insert statement. The column name must be static.
You could use dynamic SQL, however in this case you can just conditionally insert a value into the correct column using a case expression as follows:
CREATE PROCEDURE StoreTemp
(
#Temperature float
, #Seconds int
, #SessionId int
, #DatasetId int
)
AS
BEGIN
SET NOCOUNT ON;
DECLARE #Unit varchar(20), #TempCol varchar(20);
SELECT #Unit = RecordingUnit from ReadLastUnit;
--SELECT #TempCol = (case #Unit when 'Celsius' then 'Temperature_C' else 'Temperature_F' END);
INSERT INTO DATASET (Temperature_C, Temperature_F, [Time])
SELECT
CASE WHEN #Unit = 'Celsius' THEN #Temperature ELSE NULL END
, CASE WHEN #Unit != 'Celsius' THEN #Temperature ELSE NULL END
, GETDATE();
END
GO
For better performance and a neater query, you could even simplify the insert to do it all e.g.
INSERT INTO DATASET (Temperature_C, Temperature_F, [Time])
SELECT
CASE WHEN RecordingUnit = 'Celsius' THEN #Temperature ELSE NULL END
, CASE WHEN RecordingUnit != 'Celsius' THEN #Temperature ELSE NULL END
, GETDATE()
from ReadLastUnit;
Thats assuming you can always guarantee a single row in ReadLastUnit.