Retrieving the MAX Value from a SQL Column?

BlakeMcKenna
BlakeMcKenna used Ask the Experts™
on
I am trying to retrieve a MAX value from a column in a table. The issue is if the table contains NO records...I get an error saying:

"Conversion from type 'DBNULL' to type 'Integer' is not valid"

I understand why I get the error because it's returning a NULL value. I'm just not sure how to structure the statement. I know how to return a MAX value in a populated table, just not in an empty table.

    Public Function GetMaxID(ByVal tbl As String, ByVal keyFld As String, ByRef EH As ErrorHandling.ErrorHandler)
        Try
            DBConnection(EH)

            If EH.Bool Then
                sSQL = "SELECT MAX(ISNULL(" & keyFld & ",0)) + 1 FROM " & tbl
                cmd = New SqlCommand(sSQL, cnn)
                EH.Number = cmd.ExecuteScalar()
            End If

        Catch ex As Exception
            EH.ErrorMessage = "GetMaxID() - " & ex.Message & "...Contact Engineering!" & "~E"
        Finally
            cnn.Dispose()
        End Try

        Return EH
    End Function

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Jim HornSQL Server Data Dude
Most Valuable Expert 2013
Author of the Year 2015

Commented:
>ByVal keyFld As String
>MAX(ISNULL(" & keyFld & ",0))
The problem you're having here is that you're trying to do a MAX() on a character data type, which means anything other than numbers (NULL, 'banana', '1st place') is going to throw an error, if not during the MAX then definately during the + 1.

I recommend two things.
Change keyFld to ISNULL(keyFld, 0) to convert NULLs to zeros
Filter out these values by adding this to your query
WHERE ISNUMERIC(keyFld) = 1
Billing Engineer
Most Valuable Expert 2014
Top Expert 2009
Commented:
move the IsNull to outside the MAX function, and it should work ...
 sSQL = "SELECT ISNULL ( MAX( " & keyFld & ",0)) + 1 FROM " & tbl

Author

Commented:
Here is the new statement and it doesn't return anything!

Select ISNULL(ID,0) As MaxValue
  From [Temp]
 Where ISNUMERIC(ID) = 1

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Author

Commented:
Thanks Guy...that worked!
Guy Hengel [angelIII / a3]Billing Engineer
Most Valuable Expert 2014
Top Expert 2009

Commented:
please show the output of this query:
Select id, ISNUMERIC(ID) 
  From [Temp] 

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rereading the question, I think you are anyhow heading into a wrong direction.
it looks like you want to use a "IDENTITY" field instead of doing this yourself ...
http://technet.microsoft.com/en-us/library/aa933196%28v=sql.80%29.aspx

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