DCount using criteria from Form field

[Sophie Mess]

Hi,

I am looking to use DCount to see if a value (Student ID) is already in the back end table, by getting DCount to return the value of times (if at all) the student ID appears in the table.

I've tried coding it in several different ways but none seem to work... thus I was wondering if anyone had any tips?

Back end table: Student Options
Table Field: Student ID
Form Name: Option Form
Form field name: StudentID

The value i'm searching for/using look like: A762939

'studentused = DCount("[Student ID]", "Student Options", "[StudentID] = '" & Me.StudentID & "'")

any suggestions would be great, the aim of this is to stop anyone adding another record if they've already added one!

Also how could i code it that if a user had previously entered a record they could chose to amend it at a later date?

Thanks