Not_matching

Aka 'where not exists'

Signature

not_matching(left: Relation, right: Relation) -> Relation

Examples

not_matching(suppliers, shipments)

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Description

Computes a relation as a subset of left tuples for which no tuple from right would join on common attributes.

This operator is the inverse of matching, as shown by the definition below. It keeps all tuples from left but those that match a tuple from right.

def not_matching(left, right)
  minus(left, matching(left, right))
end
not_matching(suppliers, shipments)

Try it!

The synonym 'where not exists' comes from the fact that, since right attributes do not appear in the result, it may seem more intuitive to think about this operator as filtering tuples from left where there does not exist any tuple from right that would join. In SQL terms:

SELECT * FROM left WHERE NOT EXISTS (SELECT * FROM right WHERE [join condition])

Implementation notes

As for join and matching, you must take care of ensuring that the list of common attributes on which the (not) matching applies corresponds to what you want. Renamings and projections are worth having at hand. Alternatively, shortcuts can be considered (see matching and join).