I'm trying to detect if the attribute pair (A, B) is the relation key for R(A, B, C). Specifically, I want to write a query in relational algebra that will return an empty set if and only if (A, B) is a relation key. Is it possible to somehow count the occurrence of pairs (A, B) and pick only those which occur more than once?
Pair of atributes as a relation key - relational algebra
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The key or keys for a relation is defined by the schema definition: (
PRIMARY)KEYdeclaration orUNIQUEconstraint.It is not possible to tell from the data alone what attribute(s) might be a key or keys. The best you can find from the data content is that some attribute or combination of attributes is not a key -- because there are rows that contain duplicates for that subset of attributes.