Query in Mapserver

[Peter Le]

Hello,

For 2 days I've been trying to run the demo query as follows:

QUERYITEM usclass
  QUERY
    EXPRESSION "421"
    TEMPLATE lakes.html
    JOIN "data/lakejoin.dbf" islake "type"
  END
  QUERY
  EXPRESSION "^-"
    TEMPLATE land.html
    JOIN "data/lakejoin.dbf" islake "type"
  END
END # Layer

For some reasons, I always run into this error:
loadJoin(): Unknown identifier. (data/lakejoin.dbf):(119) 

Could you help me in understanding the syntax for Mapserver query?  I really
appreciate your help.

Peter Le