Select property
Use the Select property for Expression Type Query Extension or Nested Select to define the fields to add to the standard session query.
Expression Type Query Extension
The Select property must contain one single field or an expression that results in one value.
Examples:
tcmcs046.dsca
case tcmcs046.clan 
     when "ARA" then "Arabic"
     when "NLD" then "Dutch"
     else "Other"
endExpression Type Nested Select
The Select property must contain a complete query to determine the value of the calculated Field.
Examples:
select  count(*)
from    tdpur400
where   tdpur400.otbp = tccom100.bpid
select  txprc001.pric
from    txprc001
where   txprc001.item = tcibd001.item
           Note: In the 
         Where Clause of the Nested Select you cannot use form fields.