ISSUE: duplicates from UNION

[rq24] 8.2 Union matching - formal definition
The definition is unclear about duplicates.  If S is a solution
of GP1 and S is a solution of GP2, does the solution sequence
contain a copy of S for each of GP1 and GP2?  I believe the
answer should be that duplicates are maintained because they
might be meaningful to the user; if the user wishes to eliminate
duplicates, the user can specify DISTINCT. I think the best
approach would be to recognize that the UNION operator is
constructing a solution sequence from the solution sequences of
each operand.  The proposed rewording is then:

Let P be Pattern1 UNION Pattern2.  Let S1 be a solution sequence 
for Pattern1 and let S2 be a solution sequence for Pattern2.
Then a solution sequence for P is any permutation of 
S1 concatenated with S2.  

Fred

Received on Friday, 22 September 2006 21:20:18 UTC