I want to create a derived field that contains the count of the number
of rows in a DB table, based on selection criteria.

When I try to insert the count(*) code into the derived field, I get an
DB error message saying "not a single group group function", the
explanation for this message being:-

"Cause:    A SELECT list cannot include both a group function, such as AVG,
COUNT, MAX, MIN, SUM, STDDEV, or VARIANCE, and an individual column
expression, unless the individual column expression is included in a
GROUP BY clause. "

Any ideas how I can solve this?

Regards

Sue French