How do I test for DBNull so as not to crash with "Column .... does not belong to table row"? - vb.net

Question

I am reading 2 tables in T-SQL like so:
Select r.UID,c.Forename,c.Surname,c.DOB From c LEFT OUTER JOIN r on........
Then in VB.NET I loop through the dataset like so:
For Each drR In dsR.Tables(0).Rows......Next
However when I test like so:
If Convert.IsDBNull(drR("r.UID")) Then
Or
String.IsNullOrEmpty(drR("r.UID"))
Convert.IsDBNull(r.UID))
I crash with
Column 'UID' does not belong to table
row
when the second table r has no record.
I did try both r.UID and UID.
To recoup: All is fine when I have a record in the second table, but what must I do when I do not? How do I test for DBNull so as not to crash with "Column .... does not belong to table row"?
P.S. Regardng the 2 answers below: I have to test for UID so as to know whether there is a record in the 2nd table or not.

Answer 1

You need to do a few thing to get your code flying.
First consider side stepping your problem by using an INNER JOIN instead of the LEFT OUTER JOIN.
OR Split the problem and work with two sets.
A set with UIDs:
select r.Uid, c.Forename, c.Surname, c.DOB
from c
inner join r
on c.forename=r.forename and c.surname=r.surname
A set without UIDs:
select c.Forename, c.Surname, c.DOB
from c
left outer join r
on c.forename=r.forename and c.surname=r.surname
where r.Uid is null
But if you need to do it in one go with all the rows from the LEFT Table then you will need to check for DBNull and here is how to do that:
If Not IsDBNull(drR("UID")) Then
'Success
Debug.Print(drR("UID"))
Else
'Failure
End If
This is another way to do the same check:
If Not TypeOf drR("UID") Is DBNull Then
'Success
Debug.Print(drR("UID"))
Else
'Failure
End If
also if you are in a long tight loop you may gain performance by indexing the column:
Dim dt as DataTable = DAL.GetYourDataTable()
Dim ixUID As Integer = dt.Columns.IndexOf("UID")
For Each dr As DataRow In dt.Rows
If Not IsDBNull(dr(ixUID)) Then
'Success
Debug.Print(dr(ixUID))
Else
'Failure
End If
Next

Answer 2

change your query to:
Select isnull(r.UID,''),c.Forename,c.Surname,c.DOB From c LEFT OUTER JOIN r on

Answer 3

You could use the AS alias to specify the column name if needed combined with lvo's suggestion
SELECT ISNULL(r.UID,'') AS 'UID' ...etc...
In that way the column has a name and therefore would eliminate the test for checking the column name UID if it is present on another table.
Hope this helps,
Best regards,
Tom.

Answer 4

Put a break after the dataset is filled. Then in your watch window add the following to see what the column name is (assuming r.UID is the first column then columns(0) )
- i.e. columns(columnnumber - 1)
dsR.Tables(0).columns(0).ColumnName