Passing NULL value into parameterized delphi SQL server query

I am trying to pass in a null value to a TSQLDataset parameter. The query has the form: ``` Query_text:='MERGE INTO [Table] USING (VALUES (:A,:B)) AS Source (Source_A, Source_B) .... WHEN MATCHED THEN UPDATE SET A = :A WHEN NOT MATCHED THEN INSERT(A, B) VALUES (:A,:B); SQL_dataset.CommandType:=ctQuery; SQL_dataset.CommandText:=Query_text; SQL_dataset.ParamByName('A').AsString:='A'; SQL_dataset.ParamByName('B').AsString:={ COULD BE NULL, OR A STRING }; SQL_dataset.ExecSQL; ``` Parameter B is nullable, but is also a foreign key. If the user enters something in this field, then B must be validated against values in another table. If it is blank then I want it to be ignored. I was passing in '', but this obviously produces a FK violation error. I tried: ``` SQL_dataset.ParamByName('B').Value:=Null; ``` ..but then I get a "dbexpress driver does not support the tdbxtypes.unknown data type" error. I also tried: ``` SQL_dataset.ParamByName('B').DataType:=ftVariant; SQL_dataset.ParamByName('B').Value:=Null; ``` ..but then got "dbexpress driver does not support the tdbxtypes.variant data type" error. Not sure what I am doing wrong, any help would be appreciated. I am currently drawing up a parameter list based on whether the string is populated or not, and this works well; it's just a bit clunky (in my actual query) as there are quite a few parameters to validate. I am using Delphi XE4 and SQL server 2012. Update: Thanks for all the help, your suggestions were right all along, it was something else that produced that 'dbexpress driver' error. I was creating a 'flexible' parameter list in an effort to get around my problem, and this caused the exception: ``` Parameter_string:=''; If B<>'' then Parameter_string:='B = :B,' Query_text:='MERGE ...' '...' 'UPDATE SET A = :A, '+Parameter_string+' C = :C' .... ``` ... the idea being that if B is blank then the parameter won't be 'listed' in the query. This doesn't work, or my implementation of it doesn't work (not sure why, I'm obviously missing a step somewhere). Anyway, the working code: ``` Query_text:='MERGE ...' '...' 'UPDATE SET A = :A, B = :B, C = :C' .... SQL_dataset.CommandType:=ctQuery; SQL_dataset.CommandText:=Query_text; If B<>'' then begin SQL_dataset.ParamByName('B').AsString:='B'; end else begin SQL_dataset.ParamByName('B').DataType:=ftString; SQL_dataset.ParamByName('B').Value:=Null; end; ```