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Relational vs Similarity Selection (cont)

Similarity selection is used in contexts where
  • cannot define a precise matching condition
  • but can identify a notion of (S similar to T)
  • requires a measure d of "distance" between tuples
  • d=0 is an exact match, d>0 is less accurate match
  • result is a list of pairs [ (t1,d1), (t2,d2), ..., (tn,dn) ]   (ordered by di)
Uses for similarity matching:
  • text or multimedia (image/music) retrieval
  • ranked queries in conventional databases