Null

Routes one of two relations based on the contents of a third.

Usage

null (test, lhs, rhs : out-yes, out-no)

Details

If test contains zero tuples, then lhs is routed as the variable out-yes, otherwise the variable rhs will be routed as out-no.

Notes

The same named stream can be used for test, lhs, and rhs if so desired.

Known Bugs

None.

Example

Using the input:

      RELATION books: title char, author char, pub_date date, pages number
      Title1|Author1|09-01-1991|34
      Title2|Author2|12-23-1954|479
      Title3|Author2|05-09-2002|733
      Title4|Author3|01-01-1968|32
      Title5|Author2|07-03-2001|1152

when executing the plan:

      PLAN test
      {
       INPUT: stream books
       OUTPUT: stream answer
      
       BODY
       {
         select (books, "pages < 10" : selected-books)
         null (selected-books, "no books!", selected-books : answer, answer)
       }
      }

will generate the following output:

      ----------------------------------------------
      RELATION: test_answer
         attrs: dummy
      ----------------------------------------------
      no books!
      ----------------------------------------------