Cannot Populate Data Into Table.

sql = "SELECT [R.oriAddress], [R.oriCity], [R.oriPostcode], [R.oriState], [R.oriCountry], " & _
"[R.mobileNo], [R.homeNo], [R.resDate], [R.resTime], [R.surveyDate], [R.surveyTime1], " & _
"[R.surveyTime2], [R.moveDate], [R.moveTime1], [R.moveTime2], [R.destAddress], [R.destCity], " & _
"[R.destPostcode], [R.destState], [R.destCountry], [R.resType], [R.resStatus], [R.staffCode], " & _
"[S.staffCode], [S.staffName], [C.custName], [C.custIC] " & _
"FROM (Reservation R " & _
"INNER JOIN Customer C " & _
"ON R.custCode = C.custCode) INNER JOIN Staff S " & _
"ON R.staffCode = S.staffCode " & _
"WHERE R.resCode = " & id

i used sql code above to display data in a table. basically, the flow is like this. when there is a new reservation, a staff will be assigned to handle that reservation from the beginning until the end. before this, i just display a staffCode that retrieved from Reservation table.

then, i made changes to display staffName and removed staffCode from the table (based on requirement). but, because previously, there are some reservations that have been assigned to staff, i do not have any problem to view all data in the table.

but, when i click other reservations that have not assigned to any staff, an error page came out. the default value for staffCode in the Reservation table is null. because this null value, i've got an error page. is there any steps i can use to solve it?