Read-only custom field with select statement but no argument

Hello,

I am trying to create a read-only custom field  which would do the following:
Retrieve the value from a configuration table by using the user’s session ID as condition.

The configuration table’s structure is as follows:
 

I tried to create a read-only field with a select statement but arguments are mandatory.

 

Do you any idea of how to implement this functionality?

Thanks in advance,
Mélanie.