Conditional Expression - I.e., IIF In Access

I have a query with a conditional expression that I can do just fine in Access but I am having a bear of a time trying to create a similar SQL View. Baiscally I want to say, if column A is null, use value B else use value C.

In Access the SQL is this:

SELECT IIf([Categorycode] Is Null,[tblconstituents].[CASNumber],[categorycode]) AS Casnumber, Sum(qryweldingrod3a.CFume) AS CFume, Sum(qryweldingrod3a.cslag) AS cSlag
FROM qryweldingrod3a INNER JOIN tblconstituents ON qryweldingrod3a.CASNumber = tblconstituents.CASNumber
GROUP BY IIf([Categorycode] Is Null,[tblconstituents].[CASNumber],[categorycode]);


But I know you can't use the IIF statement in SQL so I was trying CASE and was still coming up empty handed. Here is what I produced in SQL but it didn't work:

SELECT SUM(dbo.RecycleWR_qryWeldingRod3a_LBS.CFume) AS CFume, SUM(dbo.RecycleWR_qryWeldingRod3a_LBS.CSlag) AS cSlag,

CASNumber = CASE Type
WHEN categoryCode IS NULL THEN dbo.tblConstituents.CASNumber ELSE CategoryCode
END,
FROM dbo.tblConstituents INNER JOIN
dbo.RecycleWR_qryWeldingRod3a_LBS ON dbo.tblConstituents.CASNumber = dbo.RecycleWR_qryWeldingRod3a_LBS.CASNumber
GROUP BY dbo.RecycleWR_qryWeldingRod3a_LBS.CASNumber

Any ideas would be greatly appreciated.