Parameter list of values with null check

Hi,

We are trying to find a solution to the following problem in a datamodel. We have a user input parameter called P_ LIST, which contains a comma-separated list of values, for example:

COPENHAGEN,STOCKHOLM,AMSTERDAM

We have a table which contains values as such:

City	Population
COPENHAGEN	10
STOCKHOLM	20
AMSTERDAM	30

The user can input one of the following variations into the parameter field:

COPENHAGEN
COPENHAGEN,STOCKHOLM

COPENHAGEN,STOCKHOLM,AMSTERDAM

STOCKHOLM,AMSTERDAM

AMSTERDAM,COPENHAGEN

NULL   (i.e. no input, should show all cities)

The SQL query is:

with cities as (SELECT 'COPENHAGEN' city,10 population from dual

        UNION

        SELECT 'STOCKHOLM' city, 20 population  from dual