Add an if() function which will enable conditional function queries.
The function could be modeled after a spreadsheet if function (e.g: http://wiki.services.openoffice.org/wiki/Documentation/How_Tos/Calc:_IF_function)
IF(test; value1; value2) where:
test is or refers to a logical value or expression that returns a logical value (TRUE or FALSE).
value1 is the value that is returned by the function if test yields TRUE.
value2 is the value that is returned by the function if test yields FALSE.
If value2 is omitted it is assumed to be FALSE; if value1 is also omitted it is assumed to be TRUE.
if(color=="red"; 100; if(color=="green"; 50; 25))
This function will check the document field "color", and if it is "red" return 100, if it is "green" return 50, else return 25.