One major function of pick lists is to ensure data entry is restricted to 
meaningful values.
Listboxes and combo boxes accept SQL to populate pick lists. A number of posts 
in the oooforum have raised the issue of directly referencing the value of 
another control in the select statement so that the choices presented are 
correct in light of data already entered.

Usually SQL expects to see external variables in the form
SELECT * FROM atable WHERE col1 = :externalvariable

In this case external variable would probably take the form 
formname.controlname